Need advice!!

So I'm dating this guy and I'm still a virgin and 18 years old. I really love him and I'm ready to let him have it. I'm super nervous for my first time because he's had experience, but I masturbate alot and orgasm every time. I would just really appreciate advice. Like, when it was your first time, what is something you wish someone would have told you/warned you about? What do you wish you knew before/during/after It? Thanks!