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
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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