Your first year of university should, to some extent, be a time of exploration.
You should consider attempting some courses that were not available at your high school. That being said, the transition from high school to university is a significant one. Choosing courses in which you can reasonably expect success is wise. It may be unwise to choose all of your courses in unfamiliar academic disciplines.
You should also give some thought to your current career goals, keeping in mind that these will likely change over your undergraduate years. If you have a career goal in mind, you might wish to choose some courses in line with those objectives and select others out of personal interest. Take some time to peruse part III of the academic calendar where you will find program information, and course descriptions.
A normal course load is 30 credits over the fall and winter terms. You should register for 10 courses in total - five courses (15 credits) in the fall term, and five courses (15 credits) in the winter term. Most courses are three-credit courses offered in one term: fall term (Sept-Dec), winter term (Jan-Apr), spring/summer term (May-Aug).