I began my career at age 16. I knew by then I wanted to work in the computer field, ideally programming, so I got part time work as an assistant to an IT consultant. He had me do all the gross jobs (like getting down into dusty crawl crawl spaces to pull cables) but he also taught me a lot about computer systems along the way as well. In college I was able to reference that experience to get an IT job on campus as well, so I was studying computer science theory, and in between helping build networks and servers in a very practical way.
If you can do it, it's great to find entry level work in the field you're interested, even if you're still in school. You might find that the career isn't what you imagined it to be as well, and be able to change direction before investing too much of your education in a certain direction.