I think everybody answered most things, but here's my 2 cents worth of info

They don't like direct sunlight, it's bad for them
They don't like places that are too small, or sometimes even too big (well, when they are alone)
They love live foods, although its good to feed them betta pellets, etc.
They like temperatures around the 80's, usually, anything below that will cause them to be sluggish, they won't move around as much.
Male bettas are very agressive and territorial, it wouldn't be good to keep them with other fish, bettas are also carnivorous. However, you could try to keep a group of females together, if they are introduced at the same time. Females introduced a bit later can get some "unwelcomeness" from the current group of females. But if you decide to keep females together, it's good to make sure they have enough room to claim their own piece of territory.
Other than that, bettas are fun to have!! I know when I got my first betta I was so excited. They are pretty hardy, and don't require as much maintenance as a goldie does (Goldies, we still love you!

) And they can be a breeze to keep. Good luck on buying your betta!!!