Many of the knots shown on that site are basically the same knot with different names and many are not technically knots, but hitches! That is unnecessarily confusing for a beginner.
My advice would be to learn, ie, a Bowline, a Figure 8; a Sheet Bend; a Round Turn and Two Half Hitches, a Clove Hitch, and a Rolling Hitch: These are the most useful of all knots, bends and hitches and introduce an understanding of how they work!
(I once had an American tell me that a Bowline was a good old Texas cattleman's knot. I said that I thought a cow had horns, which totally confused him and he went off muttering something about godamm Limeys!)
The Reef Knot, although very useful, is a misnomer. It is a bend which should never be used as such!
Useful variations are the Blood Knot which can be used to join lines together and a Grinner Knot for attaching to a fishing hook or a ring.
Cheers.