If you take more than three post-secondary courses, then you are admitted differently than if you had applied from high school. There a various processes you have to follow, depending on how many courses you have taken, and whether you are interested in General, Specialization, or Honors. In the most common situation for students who have not had a full year at university, you are admitted to the General degree, and then apply during your first year to the Specialization or Honors.
If you take 3 or less post-secondary courses while in high school, you still get admitted as if you are a regular high-school applicant. Thus you can take computing courses through a distance education school, like Athabasca University, if they are not available at your local high school.