You will need rock, sand, salt water mix, a hydrometer or refractometer, thermometer, heater, pump, power heads, lighting- if you want corals you need better lights than if you did a fish only with live rock-, saltwater testing kit...
First decide what tank size you want and then you can start getting the supplies. If you do a 10 gal
tank there are plenty of nano fish to pick from. Www.liveaquaria.com
has alot of info on alot of fish.