Okay, the last one isn't as essential, unless you happen to be named Rambo and are forced into the hills by a malicious sheriff, but you really should know how to tie a tie.
I'm going to teach you how to tie the two most basic knots, the four in hand and a mystery knot that I thought was a windsor, but which isn't. It is however better than both the half and full windsor, as it's more symmetrical. Upon further investigation, it seems closest to the Pratt knot, but unless someone can tell me differently, I'm just going to call it the Jeff knot.
Step 1: The basics for both knots.....
If you're right handed, you'll probably want the thin side on your left. If you're left handed, the opposite.
The wide end should hang lower than the thin one. How much lower is going to depend on the length of the tie, and on your height. Ultimately you'll get the feel for it after bit of trial and error.
Step 2: Four in hand: the beginning
Step 3: Four in hand: up and through
Now take the wide end of the tie, and push it through the front part of the loop. When you're done, it should look like the last picture.
Step 4: Four in hand: fishing touches
Step 5: The mystery knot: the beginning
Cross the wide side over the thin side, then bring the wide end up behind the thin side and over.
Step 6: The mystery knot: make a loop
Step 7: The mystery knot: completing the knot
Put the wide end through the front part of the loop, and pull down.
Just as in step 4, grab the knot with one hand, the thin side with the other, and pull the knot up to your collar.
Step 8: What to do with the back side and notes on length
The back of the tie should never hang below the front. That's a fashion no-no. If no matter what you do, you can't get the both the tie to end at the belt line and the back of the tie to be shorter than the front, your tie is too long. Get a shorter tie. (They do come in different lengths.)
Additionally, the back of the tie should never be flapping around. People used to use tie pins or clips to make sure of this, but most ties today either have a specific flap for the back part of the tie, or a label you can use in its stead.