Tell a story, don't think about it. At the start of the conversation you don't need to show her you're interested so just tell her stories that demonstrate that you are a good catch. Show you're clever, or good at sport or music or whatever.

If you freeze up because you feel the pressure of them being too attractive, then something I used to do was imagine them giving head. Imagining a girl doing something like that I used to find helped me talk to them.

The other thing to bear in mind is that picking up girls isn't a one way street. A lot of guys think that they have to do everything, but in reality all the girls want to be hit on, its a massive compliment, so don't think you are doing all the work. They want you to succeed just as much as you do because at the end of the day she doesn't want to go home having not met anyone either.