Hello everybody!

The situation is like this:

My parents don't know how to speak to me, they just yell!
Since I was a little boy my dream was to become a MotoGP rider. I love motorcycles. My parents never let me start a career. I love to watch on TV. I'm always anxious to watch it on TV on Sundays. When Sunday comes, they don't let me watch. When I was 17 years old, I asked for a motorcycle and they laughed and told me an instant NO! I wanted to be a rider, they say "no!"... I want a bike, they say "no!"... I want to see a simple race, they say "no!"... I'm sick and tired of it. I'm thinking in leaving home.
They don't know anything that's going on my life. In school, they just care about my grades, nothing else.
Knowing that I'm too late to become a MotoGP rider, I will never be happy in my life. I will never achieve my dream. And my parents don't care about it. I'll always be a failure.

Help please...

Sounds like they think the only good carrer is book learning and collage grad school and a docerate degree, and they think motorcycles won't give you money. Go to them and ask them "why don't you want me to have anything to do with motorcycles?" say it calmly and be secere, and see what there response is, then say. "this is something that i love, this is important to me, and if you want me to be happy let me do this." don't ask them for a bike but tell them nothing they do will stop you from acomplishing your dream.
Ps its probably a good idea to have a career as well as being a biker you know a fallback.

Such a shame your parents are trying their best, to make sure you have a chance at a solid future ahead of you, and of course you can't see the hard work they are putting in on your behalf, because your not seeing anything but what you want. There will come a time, that you will have to fend for yourself, (very soon) and you'll understand what they were trying to accomplish. You'll be a man soon enough, and can do what you want. One question, where do you plan on leaving to?