I'm not in IB either

Don't worry, regular kids aren't at a disadvantage or anything. lol .. or if so i'll let you know after first year.
All you have to do is make sure you have some decent marks and that your top 6 is somewhat higher than the cutoffs to be on the safe side.
This year the cut off for science iwas like 87.5 I think.. not too bad. So aim for about an 88 avg next year.
Focus on good marks in English, Calculus and Vectors (not necessarily advanced functions, but you should try to do well in that too), and 2 science courses (Don't stress so much about doing well in all 3, but try), McGill also doesn't look at extracurriculars, and though it would be really nice to get involved and stuff, don't overdo it if your marks are suffering.
Basically, aim for 90, but don't worry if you don't quite make it. If you can get above 95, that's even better and gets you scholarships and rez of choice.