Clarity and persistence are the keys to accomplishing big goals.
First, you have to know what you want. What are you good at? What do you like to do? Can you turn it into a project? Will you feel amazing if this project is completed? Do you care enough to keep working at it until it's finished?
In my case I like to write. That's easy enough to turn into a project: write a book. The harder part is staying dedicated. I've written two (crappy) novellas in my life and I can tell you that perseverance is difficult. I wrote the first novella when I was ~13 and I remember that I just wanted it to be done, although I can't recall why this was such a big deal at the time. Finishing the book was my motivation for finishing the book. (That's right.) The second one was written last year thanks to the prompting of my French teacher who decided that my final project for the year would be to write a novel in French. So in this case my motivation was just to finish the hundred or so pages so that I wouldn't get an "F" in the class. I didn't think about it. I just sat down and did my daily quota.
Granted, I wouldn't feel comfortable publishing either of these books. But the point is that they're done and even if I'm not Jane Austen, at least I've proven to myself that I have the capability to write these things.
You can do amazing things. What do you want to want to start with?