None at all, I'm afraid, 15l is too small for any fish. Can you return it? I would recommend 45 litres as the absolute minimum size (the RSPCA also recommends this as a minimum), and then your options would be limited pretty much to a single betta. If you want shoaling fish then 60 litres is a more realistic minimum.
15L would be impossible to maintain, you'd have all kinds of water quality problems and sooner or later, dead fish. If you get the right gear to start with then keeping fish is much easier and more enjoyable
If you can't return the 15L then you could just plant it up, it would look really pretty with some nice aquatic plants in there.