5 gallons on up, filtered, with a heater. 78-82 is the normal range. After having one of these guys and seeing how much they swim, I would recommend at least a 5 gallon tank. They like plants so planted is the best followed by silk. Never use plastic plants. If you want real plants, you’re going to need a better light than what comes with most tanks.
I have a Fluval Edge which comes with an ok light bar for low light plants. I bought another light bar that destroys the zen look of the Edge, but I wanted more light. Marineland makes a nice light kit, but I suggest you read up on planted aquariums at www.theplantedtank.net
Plants need either root tabs or liquid ferts to be their best (and some need CO2 injected into the tank!) I use both and my plants are actually growing. I have a brown thumb when it comes to terrestrial plants.
I could write pages about lighting and the difference between reef lighting and freshwater lighting, but it’s all on that site. They have a nice plant guide too. It’s a great resource.
Buy a test kit. Keep your ammonia, nitrates, and nitrites low. If they’re high, do a partial water change (use conditioner) everyday until they aren’t. Figure out what the source of the problem is and fix it. It’s usually over feeding or overstocking. I don’t worry about pH. Steady pH is better than constantly trying to mess with it.
A lot of this is basic fish keeping.
Some great resources are:
www.theskepticalaquarist.com
www.monsterfishkeepers.com
nippyfish.net
bettasplendens.com
www.ultimatebettas.com
www.bettafish.com
There is good advice and bad advice everywhere, but don’t be afraid to ask questions.
Basically just read, read, read.
I think what Bacardi is angry about (and she is welcome to correct me) is the person whose first post on a Betta forum is: “Help!! The Betta I just got is really sick and I don’t know why!”
And then you find out the Betta is kept in 2 cups of water, no heater, and the water has never been changed because they are trying to cycle it. Oh, and they got it at a wedding reception where it was a table decoration.
I dont mind answering general questions and showing you where to get answers, and I hope you’ll take a look at some of those sites I listed. Almost all of them have a “how to get started” or a “betta 101” section. Most of the sites have forums as well.
Hope this helps. 
P.S. I don’t consider myself an expert by any means. A lot has changed in the years since I kept reef tanks and WAY more since I last had freshwater tanks and under gravel filters were all the rage. I am still learning about Bettas, but they are a pretty easy fish to keep. The misinformation passed around and the conditions they are kept in at petstores continue to dog the little guys. I learn something new everyday either about Bettas or planted tanks. It’s an endlessly changing hobby and very rewarding.
I’d love to discuss reef tanks with anyone who has one too. I fantasize about having one again, even a nano, but then I think I could just set a pile of money on fire and recreate the feeling of owning one. 
