It really depends on what you want. You can get something like a nano cube style add another powerhead and heater. Things you will need no matter what type of tank you get you will want a good test kit like API liquid test kit and a refractometer. If you don't want to top off one or two times a day get an ATO.
Sand, rock and salt.
Read up on cycling and figure out what type of fish you want BEFORE you buy your tank.