Hi Fishey,
It sounds like you are fresh into the hobby. So I'll assume you know nothing, and just list the things you will need to learn before getting saltwater fish.
Water quality (chemical levels: pH, ammonia, nitrite, nitrate, salinity, temp)
Water sources (reverse-osmosis (RO), de-ionized (DI), distilled, not usually tap water)
Type of fish tank (glass or acryllic)
Lights (NO or VHO fluorescent, power compact (PC), metal halide (MH))
Type of substrate (crushed coral (CC) or sand like for a deep-sand bed (DSB))
Filters (over-the-back, sumps, canister, protein skimmers, wet-dry, etc)
Cleanup crew (hermit crabs, snails, starfish, etc)
Fish compatibility
Fish space issues (some fish require more space for their mass than others)
Well, I'm sure I missed some things, but I list what i thought of so you can learn about them, then make informed decisions about what is right for your situation. Sorry if you know all this already. Again, I was assuming you were starting from scratch.
Welcome to the addiction,
kris