Depends on what you anticipate you might need to put in it. Personally, I would go for a 20-30 gallon, especially if you may need it for larger fish like
angels, or ones that like a little swimming room, like tangs.
Rest is simple. Hang on the back (HOB) filtration, heater, powerhead and some
pvc pipe or simple decorations to give the fish someplace to hide. If to be used for fish only a simple light..if light requiring corals then maybe a T5 light fixture.
Paint the outside surface of the bottom, back..and maybe even sides with blue or black paint....OR..cover these areas with dark construction paper
Cycle the tank. Many will use an old filter from their DT to help add beneficial bacteria.