How did you know what you wanted to do after high school?

I looked in to my interests. I was interested in mental illness, so I went to college to study psychology. Then I worked professionally at it for a couple years and realized that the actual practice of it is not nearly as interesting as learning about it. Now I work part time making pizzas while I work on getting certified to teach. So, my advice? Just do something. Anything. It doesn't really matter. Life experiences will lead you where you need to be. In the meantime, just avoid sitting on your ass and you'll be fine.