A square knot is a knot used to tie two ropes of the same width together. While the square knot may be limited to ropes of the same width, it is one of the most practical knots. It is much quicker and easier to tie than most other knots of the same purpose while providing decent (not great) strength.
You will need two ropes of equal diameter (otherwise you can use both ends of the same rope). Ropes of different colors are recommended for beginners since it makes it easier to distinguish between them. Now to tying ...
Place the two ropes one next to the other. Then place the rope on the left (yellow) partly over the one on the right (black & white).
Now flip the right rope (yellow, originally on the left) under the left rope (black & white, originally on the right).
Then pull the right (yellow) rope over the left one (black).
Next, flip the left rope (yellow, it was on the right before we put it over the left one) under the right (black & white) one.
Now pull on both ends of each rope to tighten your square knot.
Here is a simple memory trick for remembering how to tie a square knot: "left over right and under, right over left and under."
If you often find that your square knots don't look right in the end, they are very likely granny knots which are tied "left over right and under, left over right and under "rather than "left over right and under, right over left and under." Here are images of both knots for comparison
Once you master the square knot, try the two half hitches.