Really an advice is based on what size of tank you want. But the basics are all the same as larger tanks.
you need:
filter
skimmer (optional, but recommended)
Heater
Someway of moving the water (tank size dependant)
lighting
You would have to watch water levels closer than you do for a larger tank. I have a 20 gal and do 1-2 water checks a week. I am not sure if that many are required, but i like to do it so i do
!
Pretty much other than that, do a lot of reading on here. The Conscientous Marine Aquariast is a great book to get started with. I also read Saltwater Tanks for dummies.
Ummm..... i know i am forgetting a lot of stuff, but i am still relatively new to the hobby myself. I am sure others can help you more than I can.
Hope i got you started a little bit
Sean