1) take back all animals to the store if you can.
2) Buy 1 lbs of live rock for every gallon of water in your fish tank
3) get a protein skimmer like a cpr, remora, or seaclone...all are hang on the back of the tank skimmers and are easy to use.
4) make sure you have 3 inches of either sand, crushed coral, or arragonite.
5) temp about 78-80 and salinity 1.024-1.026
6) Never use tap water...go to the grocery store and get either distilled water or whats called reverse osmosis water. Sometimes the pet store will have pre-mixed water, this is even better. Since you already used tap water just leave what you have, but from now on keep the latter in mind.
7) Get test kits for amonia, nitrite, nitrate, and KH. Ask the pet store what these tests are for...its all about the cycle everyone talked about.
8) get a powerhead or two to blow water around so there is circulation.
9) wait two weeks and test water conditions...if good add fish!
I think thats everything...be prepared to spend some $$$..just do the best you can.