Water quality is everything to betta health. Since your tank is new I doubt that you have established a colony of good bacteria yet.
Until that happens and the tank is cycling, you will need to check nitrites and nitrates EVERY DAY. Perform 50% water changes as required. After about 30 days, the bacteria colony should be established. When you can get close to <zero> nitrites for a week straight, your in the clear. However, I recommend performing 20% water changes weekly after that and vacuum 30-50% of the bottom when you change water.
You will also need to aerate the water with an air stone and pump.
Don't wait to take these actions. Every day matters. Head up to your local aquarium store and get the stuff you don't have ASAP.
Fishkeeping isn't a lot of work, but does require daily attention to stay ahead of any issues. Bettas make great pets due to their intelligence and interactivity.
I just wish I could take mine on a walk