What would you tell yourself? Would you give advice? What advice would that be? Ask about your perception of the world? Give us an idea of who you think you were or who you think you'll be in the future...
My late teens and twenties, even thirties were great. I worked hard and played harder. I had a lot of good friends and quite a few great ones. A few of those still hover in my life today (thanks Facebook).
But to the child I was in the 1970's I would say the following:
Don't sweat the small stuff, life will throw some really big stuff at you and the small stuff is so unimportant.
Tell your family, especially your sister, what they mean to you. The lesson you learn about how fleeting life is will come too late to amend this.
Fight for your dreams, they are worth it.
Enjoy being a child, being an adult is really hard.
The people who are worth it will always be so, the ones who treat you badly may well grow up to be wonderful people, so don't write them off forever either.
Be proud of who you are, you are an incredible person in your own right.