Oriongirl did good on the basics and I don't think I could add more to that than my own personal experience. My first saltwater tank (my 9,999th tank overall!) was a 15g nano. Here's what I did (keep in mind I have kept tanks for eons and can only say that learning the hard way on cheap freshwater stuff saved me TONS of grief and $$$ on the nano reef!). And DO call the local water supplier (if you don't use a well) to get a water analyses. ANY present nitrate, phosphate, or silicate will present a problem (Amquel Plus will remove some nitrate if it's your only issue).
1) Set up tank with good filter (150gph or better, like an Aquaclear), heater, and light (you need at least 30 watts of light, preferably compact flourescent, 50/50 spectrum). Mix up salt in seperate container and get spec. gravity right, add to tank. Get everything running without any rock or sand for a day or so (just to get dissolved oxygen in the good range). If you can pirate 10 gallons of good, clean, saltwater from a friend or LFS with a HEALTHY reef tank to start yours off with, so much the better--- full of microscopic and bacterial life that you want!
2) Add (maybe) some live sand-- I'd personally only put a half inch, but some folks like to add sand six feet deep
. Maybe 5-10 lbs? Splurge, though, on the rock. Buy the nicest looking live rock you can, and load the tank with it making a nice sloping, open (ie, not compacted) pile.
3) Add several hermit crabs that are reef safe (no expensive ones!). Hermit crabs will provide plenty ammonia if fed *lightly* to cycle the tank. Give the tank a solid month to cycle. Throughout this time use an ammonia and nitrite test kit. When both levels are 0 (ammonia will drop first) then you can add a few more crab-like inverts or maybe a few shrimp (candycane/peppermint shrimp are fantastic nano shrimp).
Water change every month or so, 20%, or do this every two weeks if you wind up with a problem with nitrate (in a nano this is very easy to control with profuse macroalgae growth).
