The Trucker’s Hitch is a great knot for tying things down (tarps, canoes, etc). Here is a step by step visual reference on this knot. I like to finish mine off with a couple half-hitches to lock it down.
Knots: The Trucker’s Hitch
Here is an example of someone using a Truckers Hitch knot and a couple of other knots, to tie a canoe to the roof of a vehicle: http://www.canoecanadaeast.com/do-it-yourself-tie-down-ropes.htm
Make sure to get a couple good books on Knots and Knot Tying, just in case the internet goes away!