Do what you normally do with sex with girls. But in this case, the guy should be the one who does everything. Now I don't know much because i've only had sex with my girlfriend. If you're afraid of ripping your vagina, then use enough lube to make sure it doesn't get dry. And make sure his penis isn't too big. It will hurt when he slides in and out a couple times but after that, it will be pleasurable. If you have used dildos for sex with girls, then it'd be the same except you have an actual penis. Make sure you use a condom, make sure he doesn't control you in the sex to the point where you don't want it, but he rapes you.
You don't really need to practice at sex, if your partner truly loves you, he will go with you every step of the way in your relationship to make sure both you, and him will enjoy what you two have with each other. If you are scared, tell him. And he should be willingly to not do it till you're ready. Compromise is pretty important.