When I was 18, I was in two situations like yours. I would be making out with a random hook up and ask the girl while fingerbanging her, "Have you ever given a guy head before?" then I'd pause if I didn't get a reply, then I'd say, "Do you wanna try?". That worked for me.
If they said no, then good old fashioned begging works too..
My first girlfriend at that age didn't go down on me for a few months into the relationship. Finally, after I went down on her and made her *** for her first time, she returned the favor.
I'm assuming the girl is around your age. If so, you're going to need to give her some guidance and reassurance in bed.
EDIT: The guy's response above it good too. You'll need to guide her hand to your wang if she's being overly passive. Help her out by holding your hand over her's and start her hand moving up and down.