1. Save yourself for marriage.
This would definately be one for me too, but I would have different things to say at different ages.
In my early teens, I would tell myself it's way too soon to be thinking about boyfriends. You can like boys and think they're cute without it going any further than that. Enjoy being able to do that at this age. It won't always be proper for you to do this. Above all, guard your heart. Jesus is the true love of your life, always.
In my older teens, I would repeat, guard your heart, even more fiercely now. Song of Solomon says, do not awaken love before its time. Serve God with all your heart, mind, soul and strength, and when He is ready, when He knows you are ready and your husband is ready, He will bring you alongside each other.
I would also include something I didn't know until after I blew it. Physical intimacy before the marriage will make you have to work twice as hard for emototional intimacy after the marriage. God's way is always, always, always better, and sometimes you don't know what or how bad the consequences of sin will be until it's too late, and that goes for every area of your life, not just this one.
That's just what I'd say to myself.