i guess bushart and reefrunner already covered all the stuff you need in the boat.
As for trailer plate go, you'll need to go to an ontario drivers licensing centre and register for one. You'll need a Bill of sale (ask the seller to write one up), proof of ownership and you'll have to pay tax for the price of the trailer (you can ask the seller to just write $50-100 on the bill of sale). Do up a search on OFC, there's some info on here... i'm going through the same process as well, have to get mine licensed this week.
Is your boat motor bigger than a 10hp? If so, it will need to be licensed. Go to Transport Canada to do that.