I would suggest researching the nitrogen cycle and how to cycle a tank without fish. Since you do not have your betta yet, I suggest holding off on getting one until the tank has been properly cycled. This will mean a lot less work for you and a lot less stress on your betta.
If you do not have this stuff already, you will need a 50watt adjustable heater, approximately 10lbs of substrate if you choose to use it, a small power filter or canister filter, a small air pump and airstone to help speed up the cycling process, a master test kit, pure ammonia from the hardware store, and some kind of biological starter media from another tank or bottled bacterial supplement. If the tank came with incandescent bulbs, those will have to be replaced with fluorescent bulbs. It is a good idea to put your decorations and live/silk plants in during the cycling process so that they can be colonized by the bacteria.