I have a 5 gallon mini bowl, with under gravel filtration w/ air pump, a small heater (50 watts I believe), thermometer, gravel and decoration in the form of a rock that he can swim in and out of.

I use Doc fishwell's fresh water aquarium salt and a water conditioner. You'll need betta food (small pellets) and they also love frozen blood worms.

That's pretty much the works. You can do it smaller and cheaper, but that's what I have.