There are three rules that every kid should know that will make their lives easier and save their parents a lot of grief.
1) There are two kinds of things in the world, things you want, things you need. Learn how to tell the difference.
2) Never do anything just because "Everybody else is doing it". If a million people do a stupid thing it still is a stupid thing, it's just that now a million stupid people are doing it.
3) Learn to think further than 10 minutes in the future. A really cool thing now might prove to be really stupid tomorrow, that is if there is a tomorrow.