You need to spend your time focusing on helping her to become aroused... put the time in with foreplay and as she becomes more excited her vagina will naturally lubricate itself and you should find it easier to penetrate her...
If you learn how to make her orgasm using your fingers and tongue first, she should gradually start to relax and you should both be able to enjoy sex more, if you're not sure how to finger her correctly (don't worry most men don't!) try this technique:
Firstly, don't worry it's bloody scary the first couple of times! Secondly, the secret to performing in bed comes from understanding a woman's body and knowing how to stimulate it...
If I were you i'd spend some time really reading up on female erogenous zones and then think about how you can satisfy her before having sex... think about it... if you have a problem with stamina, that only affects your penis doesn't it? What about your hands and your tongue? Why not work on those techniques... here's a tip...
Fingering:
1.) Place your fingers palm down so that they are pointing down towards her vagina;
2.) place your longest finger directly above her clitoris. If you don't know where this is, it's simple, where the two flats of her vagina meet at the top you'll feel a small raised bump, that's the clitoris, it's like having your finger between two hills (her Labia);
3.)Once your longest finger is on her clitoris draw it back up slightly, here you should feel (if you press) a short thin band of muscle which connects to directly to the clitoris;
4.) Start rubbing your fingers in a steady motion around this muscle so that just the nip of your finger touches down onto the clitoris at the lowest end of the circle.
5.) While you're doing this use your other arm to hold her against you, kiss her lips, cheeks, neck and nibble on her the lobe of her ear, all the time keeping the rythm with your hand down below (put your finger inside her and wipe the moisture back up over her clitoris and the area you are rubbing;
6.) listen to her breathing and her reactions to your fingers, heavy breathing and her pulling you tight - they're good signs, her squirming - bad sign, move your hand slightly back, you're probably stimulating the clitoris too heavily.
7.) Increase the speed and motion of your hand as she breathes more heavily... keep going and you should be able to bring her to climax...
The point is, if you learn how to make her orgasm using your fingers and tongue then she will enjoy and look forward to having sex with you. Making her relaxed and confident that you know how to handle her body correctly is the key to a successful sex life!
Now look I read this guide when I was younger and although it's another hard sell 'BS' site I found that the guide itself actually contained a hell of a lot of information which gave me a lot more confidence in dealing with a female body... It's probably worth looking at just to get your confidence up! therealplayers.com
Anyway, let me know if this is helpful or if you want a better explanation of the above technique... If I were you i'd focus on the foreplay, master that and she will start to trust you! It's tough being a guy because women even if they know you are a virgin expect you be good at this stuff! You really need to read up and learn how to make her orgasm using other parts of your body and she'll then relax and start to trust you during sex...