For a standard saltwater tank you will want 1-2 1/2 pounds of live rock per gallon to help with filtration. You will also need to decide what type of fish you want. Powerheads are also recomended for water movement. Protien skimmers are great for removing unwanted things from your water. Obviously you will want a heater. Also, depending on what you are wanting to keep you will want lights...for corals metal halide lighting or vho (very high output) lighting is usually the best although some corals will do ok under pc (power compact) lighting. For fish only you will be ok with normal flourescent lighting. You will also want to decide if you want a sand bed bottom in the tank, a bare bottom tank, or use crushed coral in your tank.
I hope this helps some. Any other questions feel free to ask.
Bradd