The aquarium will go through a cycling process, which can take up to 6 weeks. For an extremely small tank I would start with 1 or 2 damsels, as these are hardy. After the cycling, take out the damsels, and add small blennies or gobies, providing, you have the proper filtration. Some live rock will also be very beneficial.
Fill the tank with the same species of fish (with different colors and patterns, if you like). However, note some fish of the same species will still eat each other.
Salt water gobies are great because they are friendly and keep the tank clean. Clown fish are also a great add. Damsel fish are also great for a 15 gallon aquarium.
You can mix fish that are similar sizes, not large and small or the small will get eaten. Make sure you don't over crowd it, the rule to use is one inch of fish per each gallon of water. Don't put in agressive fish or those that can't survive in salt water.
One combination that I always enjoyed was the sea anemone and clown fish. The clown fish swim over to the anemone and wiggle around like they're scratching an itch or dancing. It's adorable!
Damsels are the best to have, especially if you are new to salt water fish tanks. The are fairly hardy fish and adjust well to new tanks. Do not put more than two in a tank, especially a small one. They can fight for their territory if they feel they are being threatened.