The Clinch Knot is quite possibly the first fishing knot that most people learn. The knot is so universally known that it is also called the Fisherman's Knot. Most fishermen know this knot and for some, it may be the only knot you know.
Despite knowing a lot of knots, I still find myself tying a good, old fashioned Clinch Knot in many situations. I use a clinch knot for bonefish when using a live shrimp. I want the hook to be tight to the line and do not like a loop in this situation. If the leader is 12-20 pound test, I feel like a 5 turn clinch is a good strong knot, but often when I am using lighter tippet like for brown or rainbow trout, I will use an improved clinch knot.
The real key to making sure you tie it correctly is to inspect it and make sure that the barrels are stacked, not hopping over one another.
Here is a step by step video on the Clinch Knot. If the video does not display for you click here
You don't really have to know 40 knots to be a good fisherman, but if you are going to learn only one knot, I suggest you learn the Uni Knot because you can use it to tie line to line as well as tying a hook to the end of the line. Click here to see a video of how to use the Uni Knot as your only knot.
All the best,