Cleaning and prep, just wipe the tank off with some water and vinegar, rinse well with water and let it dry.
Easiest paint type to use is spray paint. Cheap, fast and effective. Tape around the edges and spray it until it's coated. You can also vary the thickness of the spray to give it a different look.
The color is your preference -- I like black the best. It makes the fish and coral's colors stand out better. Some people like blue. Some people like to start with dark blue on the bottom and gradually use a lighter blue as they go upwards, blending. It's up to you.