Relax. Close your eyes, make sure you're locked in a bathroom or other room, comfortable, and calm. Then masturbate. Don't worry about the outcome, just focus on the feeling.

Russian Rocket? Sorry, it just struck me funny considering the topic.

Anyway, Eiri is right, you are probably having anxiety feelings. Just relax and make sure, as she said, to be in a place that is secure and you won't be interrupted. Take deep breaths so that you don't get too excited and do the deed.

There is no reason to be nervous because it's your body and you need to start knowing how it works. And, of course, there is nothing wrong with masturbation, so stay loose and have fun.