The first advice I'll give you is be honest with the shelter about what you think you can and can't handle, so you don't get sandbagged with a job that you can't deal with (like assisting in medical care, dealing with sick or terminally ill cats, etc.). I have a really hard time dealing with kittens, so I seldom work with them...but I'm good with aggressive cats and sick cats, so I work a lot more with them, and I'm good at reading body language and recognising sick cats, so I watch my guys closely and report issues I find. If you have special skills (photography, web design, grantwriting, connections with press and news people, special knowledge about medical issues or nutrition), let them know.
What we need most at my local shelter is cleaning staff to work the early AM shift (about 3-4 hours), because that's the bulk of the heavy work (and most of what I do). Scut work is stuff no one wants to do. Socializing happens more in the afternoons and on weekends after the cleaning is done. There's just no time during cleaning to engage in pleasantries, as we have 60+ cats to care for, and usually 2-3 people to do it. I prefer cleaning because it's harder to find people willing to do it, and I get a chance to really give the cats a quick once-over and monitor vomit, food/water use, and litterbox contents to catch health problems early, plus it gives me a really good idea of who needs special socialization work.
It is not all happy. You will lose your favorites, either through adoption or illness. Even in a no-kill shelter, euthanasia happens...if you can't deal with seeing it, be up-front about it; we usually do it at times when volunteers aren't around anyhow (I'll never forget the third day I was at the shelter, I put a box on top of a big chest freezer in the garage, and the cat manager snatched it off...I said "Geez, what'd I do, what's in there, meds?" and she told me that it was euthanized animals awaiting cremation...kinda messed up my day). If you can't handle knowing that a cat you liked was euthanized, be honest about that and ask that you not be told about it. If you have a hard time seeing animals who have been abused, be honest.
Usually the cats most in need of socialization are "problem children," and you might have to deal with some aggression or bad behavior. Ask them to start you off easy...I got tossed into the psycho ward my first day, and it was a bit of a shock.
(especially when my ankles were violently attacked by a giant, round DLH tuxedo cat without warning).
As far as disease goes: FIV should not pose a problem, as it is transmitted mainly by bites. However, URI's, distemper, and certain parasites are more easily transmitted on shoes and clothes. What I do is this: I change my shoes before I come home, come in through the cellar, throw my shelter clothes in a hot wash, change into fresh clothes, do a good hot soap scrub up to my elbows, then use alcohol hand sanitizer up to my elbows. My shelter shoes never, ever come into the house (and if I visit the shelter in street shoes, I dip them in a bleach solution before I come in the house). It may be overkill, but I haven't had a problem yet.
Seriously, though...figure out what you want to do, and what you can and can't handle, and be honest with them about it. You can always change your mind as you get more comfortable with shelter culture...I went in saying I couldn't possibly work around dogs or deal with euthanasia, and I'll soon be getting kennel cleaning training
, and I've attended several euthanasias...so sometimes, you end up doing things you never thought you could.