A CUC (clean up crew) consists of SW
snails, hermits, etc. Their job is to scavenge for left over food, most eat diatoms and some nuisance algae. They do create waste, but it's very minimal. We're talking about 5 snails say in a 30. Depending on what you think, generally people may get around 60 snails (varying species) and some hermits as well. Also if a fish or invert dies (and usually they will hide in rock so you can't find/get them) they will eat the dead animal before it decays and raises the nitrate. Nassarius are my favorite!
Ideas of CUC members:
Reef Cleaners| Clean up Crews and Macroalgae
General great online store w/ quick stats and info:
Aquarium Fish: Tropical Freshwater Fish and Saltwater Fish for Home Aquariums
First of all, what do you want in your tank? What fish/inverts are you hoping for and do you want coral or just a FOWLR
(fish only w/ live rock) tank? 30 gallon is a great starter, but that's not to say any other size will do. The more gallons the better b/c a larger volume of water allows for mistakes to have less of a consequence. If you put too much salt in a 100 gallon tank, it wouldn't have such a large effect as it would if you put too much salt in a 10 gallon. The larger you go though, the more expensive it gets. 10, 20, and 30 are sizes that many beginners have and it doesn't cost as much as larger tanks do.