Read the course descriptions
Log into Bear Tracks and use the "Browse Course Catalogue" link to read the course descriptions and make sure you have met the pre-requisite requirements for each course.
ie. Chemistry 10 is a pre-requisite for Chemistry 102. As a result you must take Chemistry 101 before you are eligible to take Chemistry 102.
ie. Biology 107 however, is not a pre-requisite for Biology 108. They are completely different streams and are independent of each other. You can therefore take them in any order.
What about Professional Programs?
Are you planning to apply for a professional program like Medicine, Dentistry, Pharmacy, Business or Education? Now is the time to review those program requirements and make sure the courses your choose work towards your goals. In many cases, it is easy to schedule the courses required for your science degree and courses required for a professional program at the same time.