First, you need to decide on a filtration system. Once you have that purchased, which you can research by looking at other threads and seeing what other people have on theirs, you need to decide on a subtrate. Crushed coral or live sand. Many people will tell you to use live sand (ls) When you first set up your tank, you will have to cycle it. To do this, go to the grocery store and buy a raw shrimp, about two inches long w/o the tail. Drop it in your water and let it rot. In order to "cycle your tank", you have to monitor your ammonia, nitrite, nitrate, and pH levels in your tank. Get a test kit like aqua pharmacuticals or salifert. All of your levels will spike first, then when ammonia, nitrite, and nitrate are at zero, your tank is cycled. Nitrates may be a little elevated. pH should be between 8.2 and 8.4.
After this, which takes anywhere from 2 -8 weeks, you are ready for a "clean up crew" which consists of snails, crabs, and shrimp, maybe a starfish. You don't add fish until your crew is in there for a week or two. Then you can add one fish at a time. Be careful about what kind of fish you get in that size of a tank. You need to be sure they can stay in that size or they'll die. Get ready to spend some money. I never realized how expensive a hobby salt water can be. I got a 55 gallon given to me for free, and I've still spent over $900 on my setup and I haven't even bought my cleanup crew!!!! Or lights that can support corrals!
.