Jenny,
You have done fine.. do not worry.
As everyone has stated, slowly let him get used to your tap water, slowly add tap water to his changed water, maybe a 1 cup of water a week. When I did research during my time at the university, I used to use distilled all the time. The minnows we raised were fine in this water.
So now the next thing I want to say is welcome to the hobby. You are under way on a marvelous trip. I can see you are an animal lover and you show responsilbilty with your pets. This is a very good thing. You will have some ups and downs in this hobby but do not be discourage. We all go through this at one point or another. I do not want to count how many tanks I have experienced in my life that has crashed for one reason or another.
So remember a few basic few things.
there is no stupid question when starting. so always ask, we do not mind.
Do your research, go out and buy a few books on bettas and tropical fish. Learn as much as you can, and then learn again. I am always learning.
Heck, if you lived Minnesota, I would probably donate Sushi a spare small tank I have. (cause its good karma)
Jim