You're certainly good to apply for admission first, which generally requires a good statement of purpose. You have to convince the admission committee that you will be able to finish the programme and get the degree.
When you have offer(s) in hand, you need to figure out how you can convince the visa officer that you can do what you're supposed to. Again, you need a convincing explanatory letter, which most people call it a "study plan", to show that you're a bona fide student who won't be violating the Canadian immigration laws. Also, you need to point out the reasons why you don't pursue your studies back home, and why your high school grade was so poor. Unless you can explain these points well, you seem suspicious to visa officers.