Questions about applying to graduate school?

When applying to graduate school it seems that you must submit your application form in Jan or Feb of the previous year. So what about all of your marks for that final year - don't they want to look at your completed marks (which wouldn't come out untill April or May) since the last year is the most important year? Also, do I need to seek out a professor who will oversee my research before I even apply to a graduate school? I live in Canada if that makes a difference