The first decision you should make is if engineering is right for you or not. If so, then try going through this website:
The Sloan Career Cornerstone Center
It has descriptions, interviews with people in the field, etc about each of the engineering disciplines. If you have more specific questions, there are plenty of knowledgeable people on this board here from each of the major engineering fields who can answer them.
Oh, and just because you pick engineering now doesn't mean you're locked into it for the next 50 years. You can always change majors while in college, and even if you graduate with an engineering degree, you still have many non-engineering job opportunities.