I sometimes ask myself what I would say if I were allowed the opportunity to travel back in time and speak to my teenaged self, but were only allowed around five minutes to do so before being yanked back to the present. I suppose I’d tell him this:

Everyone is right, you’re wrong. Now start bathing everyday, not just on Sundays.
You’ll never be a rocket scientist, but you’re no dumb ape either. Apply yourself, do your homework and complete your assignments on-time and to the best of your ability, get the best grades you can get, and strive towards graduation.
Don’t sit on your lazy ass watching shitty episodes of Mutant X and say to yourself that a job is something you don’t have to worry about now 'cause it’s a concern for the future. Before you know it, the future will be now and then where will you be? Stuck living in a decaying shithole with a family you have nothing morally, spiritually, artistically, or intellectually in common with. Grown a pair, seek out a job, and prepare yourself for adulthood.
You have homosexual tendencies within you. Don’t get all fearful and hateful; don’t become a loathsome homophobe. Accept those traits for what they are – a minor tinge in an otherwise wholly unshakable heterosexual make-up – and move on.
You like a certain girl? Show her you possess sensitivity, thoughfulness, intelligence, and most of all respect for her as a woman and as a person. First impressions matter; don’t fuck up what can be a beautiful relationship – platonic or romantic – right from Day 1.
Don’t allow your parents’ religion to define your reality, your morality, or your concept of God. Let the teachings of Christ and the findings of modern science inform you, not outmoded stories of gardens and trees and arks and floods and parting of seas.