There's no set way to do this, but if it's a first time, I'd suggest you ask her for a coffee or to go have lunch with you at a casual restaurant, like a fast food joint:
"Hey, wanna go have some coffee? / Hey, do you wanna get some MacDonalds with me?"
It'd be a nice friendly, casual atmosphere that she won't feel nervous, and it's nice just having casual conversation and getting to know each other before going on a more serious date. But if that's what you'd like, I'd say first ask her what she's free anytime on a weekend and ask her for a date. For a first one, you'd have to choose the activity.
"Hey, are you free any time this weekend?" "(girl says something)" "Oh great. I was wondering if you'd like to have dinner with me sometime / go see a movie with me / etc."
Hope this helps and good luck. Be respectful towards her and if she rejects, don't seem too sad or dejected. I for one enjoy being treated with respect and I also like men who are confident enough not to be too affected by a rejection.