Pilots are officers so that means you'll need a university degree. So your marks need to be good enough to get into university. Strong math and physics skills are essential but any degree will do.
Half of the cockpits in the CF are helos so it's not all that hard to get a helo slot. As for the jet spots, it's just timing when you go through Moose Jaw, sometimes there's 3 jet spots per course, sometimes there's only 1. The top student on Basic flight training in Moose Jaw will get their choice, after that, it's a mystery how they pick the slots. I have no idea how they do it, I think they do try to give you your second choice if you don't get your first, but it's not a guarantee. They'll send you where they need you.
Cheers