If you don't have filter yet you can use aged water. When you change your betta new water temperature has to be the same as his water so you don't shock him with temperature difference. So i advice you if you have a gall from spring water put water conditioner and let it sit for about 24 hrs before you change him.
If you keep him in his container that you brought from store make sure you keep the lid on because bettas jump. And also when you will put him in the bowl make sure you live space between water level and top of the tank.
You brought him from the store if water too dirty you can make new water. Spill out his dirty water and add a little bit of the new water. Let him sit in that water for 5 min then add a little bit new water again - do it for about 3 times so he get used to the new water temperature.