That's good you have the heater. The big deal with Betta fish is they are from tropical water and like it warm - around 80.
They like meatier foods - you could pick up some dried bloodworms - tubifex - and I believe I saw food by Tetra made specifically for Bettas as well that you could mix it up with. Do not overfeed. Feed just enough - probably a small pinch. Removed any uneaten food with a net after a few minutes. I'd probably feed the fish once a day.
Betta fish like soft water - a pH around 7. If you don't know what your tap water is, you could pick up a pH kit. If it's really high pH, then you could always just pick up a couple gallons of bottled water at the grocery store as it tends to be pretty soft.
I'd do a 50% water change once a week. If you use tap water, make sure you condition it first with a decholorinator and try to get the temp the same as the water in the aquarium.
And, of course, one betta fish per tank as they seem to like to attack each other.
Hope that helps.