Hi,welcome to the forum!.
It's nice to see that you are reading about Bettas before you get one so you can look after it well.
As a beginner I would recomend something like a 5 gallon (the more the water capacity,the more forgiving it is).If you get a 10G like you originally said about getting,you could add some tank mates e.g. a small shoal of non fin nipping fish,5 white cloud minnows or pygmy corys or some shrimp (if the Betta doesnt eat the shrimp
) plus with a 10G you could divide it later and get a second Betta etc.