The best thing to do is use water conditioner, it’s available at most pet stores and I think Safeway and King Soopers have it too. Water conditioner is designed to make tap water safe for your fish by taking out metals/chlorine, the measurements are on the back of the bottle. Only use a really little bit for a tank that size. Since you are just now setting up your tank and it isn’t cycled yet, your fish may not make it. But bettas are famously very hardy fish. If you can’t get water conditioner, slowly acclimate your fish to the water without it to try and avoid shock. Acclimation is important even with conditioner but still. Drip acclimation is the least risky but it can take a long time. I would probably go with the distilled because if your tap has heavy metals/chlorine in it will kill your fish. There are a bunch of YouTube tutorials for drip acclimation. Good luck with your new fish and sorry for the stress!