Really useful knots:
Bowline (or doubled-fig-8) - fixed loop
Zepplin bend (alternative to reef/square knot)
Prussik (friction/adjustable knot)
Clove Hitch (tight loop)
Alpine Butterfly (fixed loop mid-line)
Additionally - International Guild of Knot Tyers - Surrey Branch - The Surrey Six (surreyknots.org.uk)
This is a set of six knots that are very useful to get started with - the only additions I would put are the Trucker's Hitch and Taut-Line Hitch, and knowing how to marlin-spike a slipped version of any of the hitches.
Some additional help: (6213) 10 knots you NEED to know in life. [Save this Video] - YouTube