DanG has some good advice...try to START with good grades, since they will probably only get worse...
I went through the same problems andrzejpw...
I was a junior in high school and figured I might as well go Computer and Systems Engineering. I had been a computer dork forever, but I was kind of outgrowing it. So I got here to RPI and took my general courses (They're really not that general, the humanities requirements are VERY short). General core engineering requirements go on for about the first 2 years. Here its more like a year and a half and you start branching out. Core requirements ARENT that bad, but I know it depends a lot on your school. If I went to Oregon State it would be awful and much like what kelly describes (I have plenty of friends there). A lot of them are very cool classes with some awesome labs - the only bad ones were the math requirements, but you can't do too much about that (DanG knows how much math sucks, he's a freakin math major). Anyhow, I'm basically done with those now. In those requirements I got a good taste of a lot of different kinds of engineering. A little materials science, some computer and systems, some electrical, some civil, some biomedical, etc etc. This was really helpful and led me on my interest in civil engineering (Specifically structural engineering). I start in on some major concentration classes in January, and I'm really looking forward to it.
The way I decided was a program here called Engineering Discovery Week. Basically, for a week each department ran a presentation about the major, outlined the classes, and had alums come and speak about what they do in the workforce. It was REALLY helpful for me. From there, I scheduled meetings with the chairs of the departments I was most interested in. They helped me with my final choice. Not to mention getting to know the chairs can help a lot (they have lots of connections!).
They really do expect you to change your major at least once...I only know a few of my friends doing the same thing as they started with.
Your best choice is to pick a school with very solid programs in all of the majors you're looking at - from there just enjoy the classes and see what is most interesting to you.
And don't forget, drink a lot. You don't have to do every drug under the sun, as long as you make up for it in drinking.