I love Minibows for Bettas... they're cheap, and they come with EVERYTHING you need besides the fish, gravel and plants. My Bettas all enjoyed
this 2.5 gallon version, or you could get the 5 gallon for $10 more. Walmart has them for cheaper! It comes with a Whisper filter that I keep on the lowest setting for minimal current. Bettas don't like current!
I got a very small tank heater (at Walmart, for $5) for the cold months. In the summer, our house is 75 degrees which is perfect for Bettas. Get a temperature strip to make sure the heater is keeping it between 70-80 degrees.
Besides that, throw some aquarium sand or fine gravel in there, and a little bunch of live plants. Java ferns & Java moss are ideal, because the aquarium light for a few hours a night is plenty to keep them thriving. Live plants look much better, and add oxygen to the water. I always give them a hiding spot too. You can buy a tiny terra cotta pot for about 50 cents from Walmart and that's perfect! I find that they like checking out marbles too. Get a bottle of Stresscoat to treat tap water.
Petsmart has some awesome Bettas now!!! At least in my area. They sell gorgeous halfmoons, super deltas, plakats, etc! I don't prefer Veiltails (Bettas with tails that hang down) because they tend to get weighed down by their tails.
For food, buy a package of frozen bloodworms, thaw out a fraction of a cube per day, and feed the Betta 5 worms once or twice a day. You can also buy Hikari Betta Bites for convenience, or another pellet food, but fresh food is always best.
Most importantly, do not overfeed! Uneaten food at the bottom of the tank is all bad. Feed him only as much as he'll eat in a couple minutes and feed slowly so he can get it all. Watch for his belly, if it's bulging, that's too much food. Change the water 50% weekly or 100% every 2 weeks.