Well, Bettas are naturally aggressive, so they should only be placed in a tank alone. Also, I think a 2.5 or a 5 gallon tank is good for one. They absolutely LOVE plants, and I would throw in a hiding place. What I do is feed mine a pinch of betta food every morning. If you don't have a filtered tank, I would suggest that you clean their cage at least once every two days, but if you have a filtered tank you can clean the tank once a week. One last thing: Do NOT ever leave a mirror in front of their tank, because they will go ballistic and beat themselve's to death.
Good luck with your fish ^^