I would say that you should quickly think of an activity that you would like to do together with her to build up comfort and rapport with her (examples
http://www.datingclass.com/dating/good_places.shtml), check what time next week is good for you, the good days are tuesdays, wednesdays and thursdays. Then call her tomorrow on the third day and start of with quick general chat. Then ask her out. Like it's no big deal. She can shit test you to see if you are needy but keep a good frame. That is basically it. Good luck!