Regular tap water is fine to use for your betta as long as you use some sort of water conditioner that removes chlorine and heavy metals. I recommend Prime by Seachem.
Ideally you want a tank that is at least 2.5 gallons. You will also need a heater and a thermometer to keep the water between 78-80F (that is my preference anyway, some say higher or lower).
Doing frequent water changes of 1 50% and 1 100% a week will ensure your betta will stay healthy with clean water.
All of these things will make him live happily ever after