My advice is to just go out together and see how it develops. You don't want to come accross as being really forward. It's going to be shocking for her and I think it's best if you take her out somewhere really nice and maybe give hints to her.
For example, go somewhere that you normally go so it doesn't seem "full on" or intimidatiing for either of you. When you're out just maybe flirt a lot more than you already do, or just test the water. Casually ask her whether she's interested in anyone and leave her guessing.
You might find she likes you. These things take time. I've rushed into relationships before and those are the ones that end in tears. Being friends first is a good thing. Be patient with it. Don't go running in. It will probably scare her off.
Just try to let it evolve naturally. You can't force things, but you can certainly try to tweak things up a bit.