1) Knots I use the most:
*tautline hitch/prussic knot - to tension tarp lines
*two half hitches - on stakes (I put the tautline hitches next to the tarp so I can adjust them without going to the stake.
*sheet bend - to join two lines
*figure-8 - to join lines, as a stopper knot, as a trucker's tie-down, to form semi-permanent loops.
For hammocking, I use the Hennessee hitch or lineman's knot because it holds with slick lines and puts less stress on the line.
It is easy to be intimidated by knots. Sort of like mathematics. However, there are really only a couple of knots. Everything else is elaboration. It is all in how you shape them.
A knot (or a hitch, bend, etc. - the terminology is less important than the skill) is a way to make cordage hold on to something and let go when you want it to. In other words, good knots are simple and easy to tie, do their job, then come undone so the line can be used again. (Some would add that a knot should not weaken a line unnecessarily, but that is a more technical matter.)
Tying a knot is more than tangling line. It is also shaping the tangle into what you want. For example, a bow knot (for shoes) is just a square knot - which is just two simple overhand knots made in the right order and shaped. The bow assists in untying, but does not change the essential nature of the knot. The bowline is the exactly same knot as the sheet bend except it makes a loop on a line instead of joining two lines. You can tie a sheetbend using the same method you like for tying a bowline. The tautline hitch is a variation on two half-hitches with an added turn to increase friction on the standing line. And a half hitch is just an overhand knot seen from a different perspective. Two half hitches is where you do it twice. Big deal.
One important trick to turning a tangle into a useful knot is to give the line a little twist between your fingers as you tie it. People who get good with knots learn to do this instinctively. Try this with your shoe laces: Make the first overhand knot while rolling the lace one way, then untie it and tie it again while rolling the lace the other way. Rolled the right way, the knot will lie flat and firm. Rolled the wrong way, the lace will buckle and refuse to lie right. Simple.
Knots work through friction. Slick materials such as Spectra, nylon and some other synthetics require special attention and often require more complex knots. When learning, it is better to practice with high friction lines. TripTease is really good for this.