Talk to people you know about what they do. Find out specifics, including what skills you have to have for a particular job. (For example, plumbers have to be good with their hands. Flight attendants have to like people!) If there's a particular career you are interested in, but you don't know anyone who holds that job--ask who you do know if they know someone who has that job.
Informational Interviewing Tutorial
Offers guidelines and step-by-step advice and information about informational interviews.
Informational Interview Questions
Lists 16 questions to ask during an informational interview.
Conducting an Information Interview
Offers advice and suggests broad subject areas and specific interview questions.