It sounds as simple as the rather complex issue of anxiety. Your last line, is probably answering your own question. You're young, you're confused, and above all, you're probably putting too much pressure on yourself. It could be because you really like the person, and/or want to make sure you're "pleasing", as well as enjoying. There could be any number of reasons that thinking about sex is sending your mind into overdrive, which in turn is giving you an anxious, even "fight or flight" response, and making you nauseous. It's easy for me to say, but really, you need to relax, and even better, try to figure out WHAT you're thinking, or even over-thinking, that's causing this response. It might help you. If you can identify what, and why, you're thinking what you're thinking, you can then start sorting it out.

Be safe, and try to have some fun,
n_b

P.S. I think that being with someone you know, and trust, can also put you more at ease. If, deep down, you're having doubts, it could be causing, or contributing to anxious feelings.