If you only learn one knot make it the marlin spike hitch. It's simple to tie and leads you right into 4 other great knots. It's much more useful than the common overhand knot which is the same knot you tie your shoe with. If you don't want to tie a knot, check out my invention here.
Here are the 5 knots you can tie with this basic know-how:
1. Marlin Spike Hitch
2. Noose Knot
3. Oysterman's Stopper Knot (trefroil)
5. Twin Bowline Bend
This is how to tie them from a marlin spike hitch and what you would use them for...