A few things (not necessarily in order of importance/chronology):
1. When you figure out which degree/program you want to apply to, check the entry requirements on Oxford's website. The academic requirements are generally the equivalent of a 2:1 degree. In the US, the equivalent of a 2:1 degree is ending with around a 3.5 GPA; I'm sure you can find out online what the equivalent is in Canada.
2. Next, you need to figure out a research project that you could do at Oxford as a graduate student. There has to be a supervisor there who could work with you on the project you outline.
3. You also need to consider funding, as scholarships can be hard to come by. I'd recommend applying for absolutely everything you can find.
4. Find out the deadline(s) and apply!