What do you do When you're stupid

I won't tell you you're not stupid, because that's not going to help anyone. If one random guy on the internet could convince you you're not stupid in just 1 post, life would be a lot more simplistic. You don't have to pretend to be anything you're not. First of all, you say you're only 17. I'd say you have plenty of time. It may not seem that way but think about it, most people live on till their 80's. That means you've got 63 years old to learn everything you could possibly want to learn.
If you feel like you're not good at anything, you could try out some new activities. Pick up an instrument or try a sport. Learn a new language. Remember to keep an open mind when you try new things. The only thing holding you back is perspective.
Also, nobody is good right from the start. Maybe 1% of people can try something difficult for the first time and already be really good at it. The other 99% have to work for it. Hard. Without hard work and perseverance, no, you won't learn anything. But if you are truly ready to make a commiment you may surprise yourself.

And even if what you say is true, you try all the different things in the world and you find that you really aren't good at anything (which is impossible), why would that mean you're a waste of oxygen? Your worth as a person is not defined by what you can or can't do. It's not defined by how smart you are, how you look, or how many skills you have. Not being able to do certain things doesn't make you any less valuable than anyone else on this planet.

At the end of the day, I think what defines your worth is whatever you want to define your worth with. Lots of people enjoy helping others and even see it as a way of life. When asked, they will say it's one of the most important things in their lifes and it fulfills them when nothing else can. By trial and error you can find something which fulfills your life too.