Well, dont add anything untill your cycle is complete. Since the betta is in it already, do daily water changes, or do a 25-50% change every time ammonia goes above .5ppm so it can get down to zero. A good, liquid test kit will help you determine what it is. API is a favorite.
When yoru cycle is done, you can add some bottom-dwellers so they dont interfere with the betta. Maybe some Pygmy Cories if the betta allows it.