Oh god where to start, my granddaughter is only 3 and a half, but if she were a whole lot older, here are a few things I would want to bring up.
Don't let on line pornography warp your self esteem, don't be persuaded to do anything against your better judgement and doesn't feel is right for you. Be your own person and stand up for what you think and feel, peer pressure won't matter when you are really grown up.
The best part of the day is often the early part, try not to sleep through it, although inevitably you will when you are late teens, try not to do it for too long.
Acquaint yourself with an atlas, on line or otherwise, it's good to know where other countries are located and a bit about them.
Accoutrements aren't important they are only stuff. yeah even handbags, sometimes it's just the longing, once you get that "must have" it can cease to matter.
Don't assume your grandparents have never done anything with their lives, I'm speaking with my genealogist's hat on here,I wish I'd asked mine more, ask them about their early life and what they remembered about their own grandparents. Remember both sides of the family went into contributing to some of your traits.
Try not to ignore bank statements like I did in my youth, being an ostrich doesn't help when you have failed to rein in your spending.
Try and respect other people's opinions if they are different from yours and try to find out why they hold them.
Don't make snap judgements about people until you get to know them properly, we are all wrong sometimes.
Be kind to animals.
Don't over pluck your eyebrows in case you can't put them back. You can't have threaded what you don't have!