I think a 5 Gallon would be good, you would have space to put some decorations and the betta would be happy on it.
Most LPS (local pet store) do have My first fish tank that is around 5 gallon or more, and came with all the stuff you need, Filter, Heater and some decoration.
Then you just need to buy a good water conditioner, fish food and a water test.
After that you just need to start your cycle, fish-less or you can do a fish in cycle but, fish in cycle will require you to do daily partial water changes until your water parameters be fine.
Im not expert on this but i hope this answered you!