Even if you can figure out what career areas you like, such as medicine, law, sales, technology, etc., I think one of the most important things is figuring out what type of environment you like to work in. For instance, do like like to work alone (I do, which is why I love being a computer programmer), or would you rather work with people?
Even if you like working with people, it may not mean you like working with
all people. For instance you might find working with "the public" (such as being a bank teller) horrendous, but find you enjoy working with people one-on-one, like a physical therapist. Others might prefer light contact with many people (the public) rather than deeper interaction with a few person.
For instance, let's say you like medicine. Then ask yourself do you like noisy, fast-paced environments (emergency room doctor, EMT) or more quiet orderly environments (research scientist, medical lab technician)?
Have you ever taken the Meyers-Briggs test? It's a good tool for personality assessment and career ideas:
Here's an on-line version:
http://www.humanmetrics.com/cgi-win/JTypes2.asp
Once you take it there are links that describe your results, what it means, and what types of careers you might like.