Hey there :D
You're going to need a heater (5 watts per gallon) , dechlorinator (Prime by Seachem is really good), and if you want- a lot of decorations.
Technically, the minimum is 2 gallons. If you get a 2 gallon (plenty of space), you won't need a filter, so that'd save on money and energy. Of course, bigger is always better, so go with what you want. If you decide to get a 5 gallon and a filter, make sure your filter isn't too strong for your betta. If it is, you can baffle it by pulling up the U tube slightly, or by typing a filter sponge to the outlet. Apart from rinsing the tank and equipment down with hot water, and dechlorinating the water after you add it in, you're set to put your betta into the tank. Just be sure to acclimate him :)
Most importantly however... you should add pictures! ^-^ We love seeing pictures on here! XD