Ooooooo, a 15 gallon.....so many options!!
-You could do a community tank with one betta(male or female)and a nice school(6-8)of your favorite tetra OR you could go with a sand substrate and do a shoal(same number as tetras)of little cory catfish with a single betta.
-You could pack the tank with real/silk plants do a betta Sorority with 6-8 little betta girlies.
-You could even divide the tank 2-4 ways for 2-4 boys!
-Or if you didn't want to do bettas, you could look into a few other peaceful community fish like Platys or guppies, Dwarf Gouramis, tetras, corys, and whatnot and see what you like and what would work in your tank.
Whatever you decide to do, I highly suggest you do a fishless nitrogen cycle in your tank first. Have you looked into possibly doing a NTP yet?