Is there any advice which you trust so much that you would give it to absolutely anyone? If so, what is it? Why is that advice so good in your eyes? What in your life led you to value this particular advice?
Also don't post cryptic stuff like "don't do that thing I did once because it was bad" because that's annoying. ^__^
My advice would be to keep a positive and sunny outlook and aura about you everyday and try to spread some of that happiness to others. Its always worth it to see the smile on the face of the person you helped.
It's pretty cliché, but when you're going through a bad time just remember that it won't last forever and things will always get better, so don't let it get you down too much since it'll be over soon. :)
If there's something you really want, work toward it every day. Let people help you, advise you, but don't stop working for it. Even if you don't get it, the knowledge that you worked as hard as you could will be almost as good as the thing itself and you'll be that much better off for whatever you try next.