Here's the link to the page to start on for reserving. In searching this for you I just found out that the reservations start mid January this time around, in all the past years they have been in April! I had better get my plans in place by Christmas!!
-click the blue reserve online box on right
-choose language(annoying Canadian thing!)
-click the "MY RESERVATIONS"tab at the top
-you have to use "continue to GCKey" option as an international customer
-select english,,again!
-click on the small"register" for a new GCKey
-read the bla,bla,bla and "accept"
-fill in all the info and "continue"
-select "continue" on the next page
-fill in all your personal info on this page and hit "create account" at the bottom and they will email you the confirmation.
Then you can go back to the original link below and select the blue reserve online box again
-click "my reservations" at the top
-select "continue to GCKey" tab
-enter your user name and password and "continue"
-click "browse maps" at the top an d you can pick the campgrounds, rv size and dates etc. here.
-then hit "reserve" and you will get an email in a bit with the confirmation info and a bar code etc. print these and bring them with you to give tot he people at the campground gates so they can scan them and send you in.
Hope this helps. It's a lot of steps to get set up with the GCKey, but once it's done it's easy.
http://www.pc.gc.ca/eng/voyage-travel/reserve.aspx