Bettas need at least a 2.5 gal - preferably a 5 gal tank.
Keep the temp a stable 76 - 80oF.
Make sure there aren't any rough or sharp objects in the tank that may damage the fins - beware of plastic plants for this reason.
Bettas appreciate something to hide under and rest on top of. A small suction cup soapdish or a clump of Java moss serves this purpose well.
Hikari makes the best betta pellets, IMO. Supplement the pellets with an occasional shelled, parboiled pea to prevent constipation and bloat.
Bettas absolutely go nuts over bloodworms and brine shrimp - frozen or freeze-dried. They make a great once-a-week treat.