I'll regularly update this post to make sure that I'm not missing anything:
1. You'll need a DSLR. There's no way around that. The key is the lens (or lenses) so getting the least expensive Canon or Nikon body would be perfectly fine. My camera's from Canon so the following is Canon-oriented, but the principles are the same.
2. Speaking of lenses:
a. The correct zoom lens will be expensive since you'll need a 2.8 aperture. I'm a "Canon" person, but Nikon has the same or similar focal lengths. The ultimate Canon zoom is the 70-200 2.8 IS II. Image stabilization becomes less important as the shutter speed speeds-up so I can also recommend the non-IS 70-200 (for half the price). Many still say that the newest Canon with image stabilization is worth it, though. It has the latest-and-greatest optics and the IS might come in handy for "event" shooting where slower shutter speeds are the norm. You'll want to turn the IS off, however, for sports since faster shutter speeds negate the need for image stabilization and tend to slow down the focus anyway.
On a related note, it looks like Tamron is about to introduce a pretty nice 70-200 2.8 zoom that might be out by the end of 2012 and the one from Sigma always gets a lot of great reviews.
b. The good news is that I prefer prime lenses (or fixed focal length lenses) for indoor sports. Prime lenses are much smaller, have a larger maximum aperture (in many cases) and are less expensive. Also, prime lenses seem to produce sharper image quality with nicer contrast and a smoother out-of-focus area for "better" isolation of the player. A zoom lens would offer greater flexibility of course, but so far so good with prime lenses.
1. The prime lenses to consider are the 85 1.8 or 100 2.0 (used on the side of the court), 135 2.0 (used from the side and mid-court on the other side of the net) and/or 200 2.8 (straight on through the net from the other side and up in the stands). After a season with the 135 I replaced it with a 100 2.0 and 200 2.8. The 135 is a little bit "inbetween" on my camera so an 85 or 100 and a 200 seem to be "better", but the one-lens-solution is always attractive with all of its limitations and advantages. The 100 2.0 seems a little better built than the 85 1.8, but that and the focal length are probably splitting hairs. Speaking of a one-lens-solution definitely brings us back to a zoom, but then you have that massive lens to deal with - another trade-off (but many don't mind) and a potential image-quality hit (that some see and some don't - I'm one who does). Plus, there may be times when 2.8 isn't enough for a poorly lit gym. Some say, too, that a 2.8 prime lets in more light than a 2.8 zoom since there's less stuff in a prime.
3. My camera is a lowly Canon T1i (Canon's now up to the T4i) and I like it a lot. The next step up would probably be a 7D so I look forward to seeing what the upcoming 7D II will be all about. From what I'm told the new Canon 5D Mark III is awesome on all counts (to include for sports - usually known for landscape and portraits and not sports) and the upcoming 6D "entry-level full-frame" might be great. The Canon Rebels and 7D have a "crop" sensor while the 5D's and 6D have a "full-frame" sensor. In a nutshell, full-frames have a larger sensor that generally produce nicer images especially at higher ISO settings. Crop sensor cameras have a smaller sensor that make the image look 1.6x larger. I might be wording that incorrectly, but it's pretty much like a 1.6x effect. So an 85mm lens behaves like a 136mm and a 200 seems like a 320mm lens.
In my opinion, though, the magical image quality that we seek is more about the lens (and you, of course) and less about the camera. Also, the advantages of the 1.6x effect of a crop sensor might be more important to many than a larger sensor offered by a full-frame camera. There's constant trade-offs and multiple variables so my best advice for an ideal starting point would be the latest model "consumer" dslr and an 85 or 100mm lens.
Also, don't overlook the advanced focusing systems of the many modern designs. I can't comment from experience, but definitely will one day although my "older" Canon T1i definitely gets the job done.
4. Put your camera on the Manual setting. If you're like me this was initially scary, but it's a good thing.
a. For volleyball you'll need a shutter speed of 1/400 or faster (faster is better). You'll find yourself teetering back and forth between 1/400, 1/500, 1/640 and 1/800 depending on light.
Note that if shooting in RAW a faster shutter speed that leads to a little underexposure can be corrected in post processing by lightening things up a bit.
b. Put your aperture on the largest setting (smallest number). An 85 1.8 lens will have 1.8 as its largest aperture. You can experiment with maybe putting it on 2.0, in this case. Sometimes the image quality improves a bit as the aperture decreases (number increases), but a larger aperture lets in more light thereby allowing you to have a faster shutter speed. PLEASE NOTE, though, that a larger aperture creates a thinner Depth Of Field making the front-to-back focus area thinner. So it's more challenging to have your subject (the player) in focus. The solution is to put your camera's focus selector on a single center point of focus and keep that dot on the player that you're capturing.
Just to illustrate the point, an opposite "Depth Of Field" example would be a landscape picture where you might prefer that everything is in focus. Your aperture setting might, in that case, be between 8 and 11 (or even higher).
Another good thing about a low aperture number is that, when properly done, the player/ball will be in focus with a nicely blurred background. It's the same concept as used in professional portrait photography.
And one thing to note is that the Depth Of Field seems to increase a little as the distance between the camera and subject increases.
c. The 3rd and final variable is ISO which is the sensitivity of your sensor. A higher number makes your camera more sensitive to light. A sunny day outside might call for an ISO of 100 while a dark gym with brutal lighting conditions might call for 6400. "Graininess" increases as the number goes up, but most cameras these days do well up to 6400 and some do well even higher than that. 6400 is a little high with my camera (noticable decrease in quality from 3200 to 6400), but much of that is made up for by my good lenses. So, a gym with really bad lighting conditions (kinda like ours) might call for the largest aperture opening, 1/400 shutter speed and an ISO setting of 6400. Different positions in the gym might allow you to speed the shutter up a little as the light changes a bit with different locations and subjects. Again, faster is better (generally up to 1/800) to freeze the action to the best of your ability.
d. Have no fear if you're new to photography - your camera's viewfinder will have a little metering system in it readily showing where the shutter speed, aperture and ISO are all in balance. You may periodically go a little brighter or darker from the middle (where everything's theoretically in balance), but it's a pretty accurate guideline. You'll find, too, that there's many variations of the 3 variables that'll create "metering" balance, but you'll learn from experience and learn other variations for other types of photography (to include other shooting modes like Av, Tv and P, for example).
e. Also, for moving objects (like volleyball players) set your AF mode to "AI SERVO" and your Metering mode to "Center-weighted average". Note that your camera won't beep (when in focus) on AI SERVO like it does on ONE SHOT. I still have more to learn about Metering modes so, as with anything, experimenting is always a good thing.
5. My favorite topic is White Balance (when shooting JPEG in rough conditions). If shooting JPEG you will need to do a custom white balance (to properly calibrate the colors coming out of your camera to the conditions in the gym). It's REALLY easy and a great thing. The pre-set white balance settings on your camera are often accurate, but can be "hurt'in" in gyms with challenging lighting. My favorite traditional white balance card is the 20-incher from Lastolite that folds out (or the 12 inch version for travel). The expodisc is also worth considering. There's some great resources online for how to do a custom white balance and it's EASY.
I now shoot in RAW using Lightroom 4 to process afterwards on the computer. This way you can keep your camera's white balance setting on Auto and adjust it later on. Not a bad idea to get it right in the camera for less adjusting later on, but not entirely necessary with RAW. Most feel that the less to mess with during the shoot the better.
Learning how to post-process was a bit of a horror (for me anyway). Your camera will come with a RAW software package, but I went with everyone's recommendation and got Lightroom 4. The Adobe tutorials by Julieanne Kost got me over the hump and I'm about to view them all again. She has her own web page with all of her tutorials. Again, it was tough for a newbie like me, but a good thing once I got past the mental block.
RAW tends to slow the camera down making Continuous shooting more of a challenge. That's one of the reasons why many pros stick with JPEG. My current camera's not fast enough for effective Continuous shooting in fast sports anyway so it doesn't matter for me.
RAW, though, is particularly useful for indoor sports especially when the lighting is bad. You can even underexpose to get a lower ISO and/or greater depth-of-field and add some exposure in post-processing.
6. If you're new it's, in my opinion, a great idea to get a training DVD that's specific for your camera. A good one will eliminate a few years of trial-and-error in just a few hours. After that you'll do well to whip through your owner's manual. You'll generally benefit more from your owner's manual after viewing the DVD (or DVD's). My photography life began with the Canon T1i DVD from Michael the Mentor which went over my camera as well as the basics of photography. I viewed it a few times in multiple short sessions.
7. The final step is taking pictures. Experience will be your best teacher. Try to know the game to improve your reaction time. Maybe pick one player at a time so you're not all over the place and even sort of pre-focus on where you think the shot might be. Watch your background, too, to make sure it's not a distraction from the subject.
Try to capture the player's face and it's important to have the ball in the shot. Your autofocus can easily trigger on the net, a wall or another player, but a single autofocus point (usually the middle one) hopefully minimizes that.
Also, don't look at the picture in the camera after you take a picture. It's a distraction that takes away from the concentration you'll need to get good pictures.
My camera's pretty slow, but a good sports camera like a Canon 7D has a very good "Continuous" function. I usually like to "settle-in" and make each shot count so I'm not so sure how I'd respond to rapid-fire shooting, but many do seem to advocate it (for obvious reasons).
Volleyball's a very hard sport to shoot so do plan on taking a lot of shots. Last season I'd take 400+ to get 30 good ones (which some say is pretty good). This season I'm going to try to concentrate more intently with the goal of just a handful of awesome shots per event. I'm really just looking for that one stunning picture, but I'm sure to evolve, change my mind and figure something else out.
Processing the RAW shots is less daunting than it might seem. You don't necessarily have to individually process all the pictures. Assuming that the lighting remains somewhat consistent (even though locations and subjects don't) you might only have to process a single picture and then sync the rest to just that one. I'll generally eliminate all the non-keepers (which I've found easier to do in the camera), process one picture, sync, crop where needed and then do a quick fine-tuning of each picture if necessary.
I keep teetering between replacing my prime lenses with a zoom or sticking with the prime lenses. For now I'll stick with primes for their small size, great value, arguably better image quality, larger widest aperture/"speed" and simpler design and function although will likely get a zoom one day for outdoor sports since the athletes move around a field making the ability to zoom more important even if the image quality slightly suffers.
Good luck out there and please don't hesitate to email me at firstname.lastname@example.org if you have any contributions or critique.
A Few Online Resources:
The Sports Photographer
Big Lens Fast Shutter