Advice for Getting into Universities

I'm only 15, heading into Gr10 this year, and I've suddenly been really worried about how I will get into university. I would consider myself 'smart', getting anywhere in the 90s for all my honors classes. With that said, I am definitely not a sport type of person, and am too shy to join one (although I am trying to work out and be more confident about this). I am going to finish Math 10 with 100% in summer school, and am going to finish math courses quickly, hoping to finish AP Calculus in gr11. This year, though, I only got into Science 10 Honors (and Math 10 Honors, but I am taking Math 11 Regular), due to some mistake which I do not know of because I do participate in class, teachers like me, and I have some of the highest marks, yet I did not get in.. I won't go into any further details about this, but yeah. Is that okay for me not doing honors? Also, I am trying to join clubs and things at school, but I have yet to decide. I basically need some general advice on what I should do so that after I finish high school, I can get into a good university. I don't exactly know what I'll be studying, so I am keeping my options open. I know this seems a little misplaced in a Maplestory forum, but heck, I was just curious and I know there are many adults here. Thanks for all the help.

Also, if I live in Canada, how would going to an American university work?