Here's a list of what you need to get started:
Atleast a gallon tank - You can find a small betta tank mostly anywhere you can find a betta and they are very affordable. You can buy these in plastic or glass it doesn't matter which. A gallon is a good size even though more than a gallon would be even better for you and for your betta. Bigger tank = less water changes
Water conditioner - This is to make sure you aren't putting your betta into chlorine and other harmful mineral infested water. I use Stress Coat, there are many, many, many more out there as long as they remove chlorine and stuff like that it should be fine. The stuff I use removes all that junk, plus it helps with the fished slime coat to make the fish less stressful
Food - Of course you can't leave out the most important part, FOOD! What would your little piggy do without food? Trust me, some of them are really pigs. Bettas are meat eaters, and will it a variety of foods. You can find basic betta pellets or flakes from your local fish store. They will more than likely have dried bloodworms or dried brine shrimp. You can probably find a brine shrimp culture if you go to a real petshop, which of course bettas would much rather have live food, but I know that's out of the picture for some of you.
Ok, so now you've just gotten home from the pet store and you can't wait to plop your little fishy in his tank and watch him swim around. Well....you can't do that. So don't...
First whatever little or big tank you decided to get for your little guy make sure to wash it very well.
You need to let the water age for atleast 24 hours, but as long as you bought water conditioner he should be alright. Get your water not to hot and not to cold, and your conditioner to it and take it to the room that the tank will be sitting in. Now take your betta and let him float for atleast 10 minutes, still in his bag or jar in his new tank. If he came in a jar and it won't fit in the top of his new tank let him sit in the same room with the tank for about 30 minutes to an hour, so that his water and the tank water can become room temperature before you let him in his tank.
Don't feed him until the next morning so that he can get used to his setting and get him in regular times of eating.
Make sure to clean his tank frequently or when ever the water starts looking cloudy