Tell her you're a virgin. IF you've been going out for a while...it means she likes you...and I'm sure she'll be okay with it. She'll probably be charmed by it as well.

You can also tell her you are a bit worried about your performance.

Truth though? Having sex isn't about "performing" or how good or bad you are. Not with someone you care about. It is about communicating physically what is going on for you emotionally. It is another language.

You can start by just making out kissing and slowly lead up to the other stuff. That'll help to reduce your nervousness also.