Flying to Canada, what to expect with customs and etc.?

Flying from Grand Rapids, Mich. to Minneapolis, layover 30 mins. or so, then flying to Seattle, then layover for about an hour, then flying to Kelowna, BC. Flying with Northwest the first 2 flights and Alaska Airlines the last flight. I would like to know where I would have to go through customs, in Grand Rapids or Kelowna, and if anybody has helpful hints and advice for my first time flying. And I would like to know more about how custom's works, what can I expect? Anything would be helpful :) I don't have my tickets right now, my bf in Canada set it up through a travel agency and i just have a confirmation number (basically etickets), etc. I have no clue where I will be going or where to go till then. Little info on my itinerary. Using terminal Lindbergh in Minneapolis, and thats all I know.