Welcome to the world of bettas! They are by far my favorite type of fish to keep! :D A few things you should buy for your soon-to-be-fish would include:
-Tank (I suggest 5 gallons, it's a reasonable size, and I like it
-Heater(Water need to be around 78-80 farenheit)
-Fake, or live plants (for fake plants, get silk, plastic ones can rip fins. I have plastic ones, but they are VERY soft)
-Food (of course; people on here prefer pellets, as they are easier, to limit, and feed. New Life Spectrum, and Omega One, are both great brands!)
-A few other odds, and ends that I can't remember...