Have you bought aquarium water conditioner and fish food?
Seachem Prime is a good one for new aquariums. It can detoxify ammonia for up to 48 hours.
If you plan to get betta, I would recommend pellets over flakes. The best pellets in term of nutrition value is New Life Spectrum. Betta has a small mouth. So pick the "Small Fish Formula" from New Life Spectrum is a good idea. Those pellets are 0.5mm.
By the way, if you do not plan to do a fishless cycling, I'd recommend you to get a bottle of bacteria seeding product such as Tetra SafeStart. Use the whole bottle 24 hours before you get the fish can help the aquarium nitrogen cycle (with the filter running the whole time of course).
