I've done a few canvasses in the past (and got a bunch more waiting to start on!) and I've just taken a look back to see what I did. Like the others recommended, gesso is a great base and seems to accept all other mediums quite happily.
Looks like I didn't bother doing that on the bathroom ones but I didn't actually paint on top or do anything too 'arty'
. The other one - a small decorative piece - wasn't gessoed either and the ink did bleed a lot!
If you'd like to take a look then at my blog you can search for 'canvas' and they'll show up there.
Have fun, Joanna