If you're in science, there is a special website that will be open called scidropandadd; you can use it to switch lab/tutorial/class sections once you enroll in a class.  Although, as kenneth said, you can change it on solar during registration time (such as now 

 )
For first year, you can read online reviews from sites such as this one to see which ones are worthwhile.  There is also a course evaluation feature on mugsi.  If you can figure out how to use it, you can see the class ratings for each professor.
Good luck!
Paul