I've never had to ASK a girl to have sex with me... granted, I've only had sex with 3 girls ever (hey, I'm 19, give me a break), but it's always "just happened".

I have a simple plan you can follow...

1) Actually like the girl for who she is. Be able to hold a conversation with her. Want to be with her in a non-sexual manner. Go out on a few dates.

2) Invite her to your house and cook her a meal or watch TV together, a movie, or something, just be alone together.

3) (I'm assuming by this point you can hold her hand or put your arm around her) Arm around her, play with her hair so softly she can barely feel it, caress her neck gently. Other hand - gently stroke her arm and thigh close to her knee. If she responds to this in a good manner (she rubs you back), keep going and see where it leads. If she seems uncomfortable, withdraw and just watch the movie. Make sure you have steps 1 and 2 down if she seems uncomfortable. Eventually you'll "just happen" to kiss and things will snowball from there.

Tips: If you focus on making her happy (both sexually and non) she'll focus on making you happy.

Bonus Info: Massages tend to turn into oral/sex at a surprising rate (but don't expect it, just be happy if it does)