You don't need to know many knots when fishing but if there's one, it would have to be the half-blood. Tying it is simple. Slide the line through the eye of the hook, wrap the end five times around the line and then tuck it back though the loop between the hook eye and first of the wraps. But to make it even more secure, before you tighten the knot continue the end of the of the line through the large loop and then pull though. (see diagram below).
This effectively locks the knot so that it cannot slip. If nothing else this tucked or locked half blood should give you greater confidence when you're banged into a hard-fighting fish. More importantly, it's a must if you're using a tapered leader.