I'm a virgin, so I have a question about sex. My girlfriend is not a virgin, by the way. Anyhow, and this may sound like a stupid question, but is it hard to put it in her when having sex? Once I'm on top of her, is it easy to penetrate her?
Yes, its fairly easy to "guide" yourself in. Most likely if you are not exectly where you should be OR where she wants you to be your GF will use her hand to help, or you could use you own hand to help with the "docking"
Inserting your penis should not be a problem provided your g/f is well lubricated and ready for intercourse.
Make sure that she is fully aroused either through manual stimulation or oral stimulation first and then have her guide your penis by placing the head of your penis into her vagina. From there, penetration should go quite easily.