Start with her back. Alil higher than the small of her back is perfect, for the first touch.
If she doesn't look uncomfortable then the NEXT touch should be the small of her back.
Do it casually. Like it's nothing.
You can do it when opening the door, like everyone else said. And you could do it when you are moving in closer to her to hear what she is saying. (i.e. at bars, or crowded places, where it's pretty loud).
When you touch her back, make it seem like you are just protecting her from people who might bump into her, or supporting her and yourself so you don't bump heads when you move close.
DONT leave your hand back there TOO long. It will creep her out, and make things awkward. But Don't make it too quick either. Leave enough time for her to notice it, then let go.
This gesture makes a girl feel protected. And that is what a lot of girls want to feel with a guy.
After you pass the back touch, you will feel more comfortable with other touches. But start off with the back first. That is the most subtle, sweet, innocent way a guy could show a girl he is interested in her. For us girls, we take it as, he's so strong, and will protect me from harm :) haha
Good luck :)