Why did I write a letter to my younger me?
My favourite author and editor of First Class (a Czech magazine) Petr Cassanova wrote a letter to his younger self and it inspired me, because I started to think what would I say to my younger me? Would I give away to much, would I say don't go to college? No, of course not :-) I would write this:

Hello you!
You are already a teenager. 15 years old, that is someting, right? I know you like trying new things, you have already smoked your first cigarette and had your first shot, that's ok. 
I am not writing you to give you lecture, I want to encourage you and assure you that everything is going to be alright! Very, very soon, I promise! 
In your life, however, you will have to make many decisions and making them according to what other people think is bu****it! Do it as you feel it. If you don't want to go to college, it's fine. You can study later. If you want to travel abroad, do it! If you don't want to live your life according to what other people say is right, do it your way! People who love will always be there for you to support you. Maybe not always from the beginning, but they will come. 
You are perfect! Do everything with your heart, don't be mean, always say what you think.
Life isn't hard, it is people who can make you suffer, but it's always up to if you let them hurt you.
Be strong, do things you love, stay positive & believe.