That's a great question, and an important one. Here are some things I'd advise to my younger self:
1. Yes, you can do it. Don't let anyone tell you otherwise. YOU are the only one who can decide when you're done, but don't give up because you're tired.
2. Find good people. The topic you study or work at can all be something you learn to love but if you don't have good people in your corner, things will be much harder.
3. EVERYTHING is somebody's job. Designing lego models, cleaning world class telescopes, deciding on the color of band aids, inventing new software for medical imaging - everything you encounter in your life is someone's job which means it's up for consideration AS a job. Think outside the box, and don't let adults who don't know any better restrict you (thinking of my high school career guidance teacher who told me if I studied math, I'd have to go into finance or teaching). If you don't know someone who can give you advice, find someone. Email, twitter, forums like this - there are always people willing to help. But do your research! Figure out how to get there.
4. You don't have to decide on the rest of your life right away. Your life and career will have twists and turns and take you places you can't possibly predict. So make decisions that make sense now and keep an eye out for opportunities down the line. A friend once said, as long as every decision you make makes your life a little bit better, it was the right decision. It doesn't have to be the forever decision.
Good luck!