When I was younger, I dreamed like I had my whole life to figure that out. In elementary school, I dreamed of being some kind of superhero. I wanted to be someone who helped people every single day. In high school, I wanted to be a computer programmer and make tons of money. In college, I aspired to be a teacher, lawyer, or a police officer. They were the three professions I was most interested in. Now I'm a proud high school English teacher, but my new main focus is with being a great husband and father for my family.
The best answer I have is, "I don't know." I have a hard time concretely deciding what I want to do with the limited time I've been given in this wonderful world. It's a question to ask myself over and over. Sometimes an answer may come, and sometimes I'm just left wondering. If I do have an answer, I know it'll change. If it changes or I'm dumbfounded, it's perfectly fine. I don't think there is a perfect way to answer this question. Being "lost" is fine because it gives me a chance to explore and reexamine my priorities.
I honestly don't think having a perfect answer is important, the act of examining and challenging one's life is what is really significant.