Ontario Universities’ Info is a great place to start researching university programs. You might find it helpful to review the programs that interest you, develop a list and then narrow down your program options.
Keep in mind that certain programs have prerequisite courses that you must successfully complete (or be in the process of completing) to be considered for admission.
When you are researching, keep track of information you might need later in the application process.
For example, find the OUAC program code (a 2- or 3-letter code) for each university program that interests you and keep it close by for easy reference when you apply.