It's natural to be nervous, so don't worry about that part. They'll be interviewing you mainly to make sure you're not some kook, to put it bluntly, and to see if they think you really do have a vocation to the priesthood. Expect questions like, "Why do you want to become a priest?" "What do you think would be the hardest part about the priesthood for you?" Stuff like that.
Pray before you go, for sure. Maybe attend Mass, too, beforehand, if you can. After that, keep in mind that if it is God's will that you become a priest, then He has known this for all eternity, and knows already if you will be accepted, and how everything will turn out. God, remember, does not live from moment to moment like we do. He lives in eternity. He does not "foresee" the future, but "sees" the future as you and I see the current moment. Therefore, He currently sees your entire life, from beginning to end, all at once. Your job is to find and follow His will in your life to the best of your ability. To offer up your sufferings, anxieties, and prayers, for yourself and others. And in whatever vocation you end up, the very best advice I've ever heard was to always, always come as a servant to others.
God bless! May God be with you! :)