Some ropecraft geeks like to start out newbies with the really simple knots; overhand knots, square knots, etc.
I think that the most useful knot, and THE knot that you should start out learning, is the Figure Eight knot.
The Figure Eight (IMO) is THE foundation knot to learn. Once you get it down there are many other variations of it that you will use time and again in climbing and rappelling. Everything from; setting up anchors, tying into harness, splicing together ropes, making stoppers to forming attachment points, will involve the Figure Eight.
Perhaps the most important characteristic of this knot is the ability to untie it, even after it has been set under a heavy load. An overhand knot can be set so tight that cutting it out could be your only option.
If you are really interested in learning this stuff I suggest getting yourself a length of cord that you can coil up and carry around in your pocket. If you are ever sitting around waiting for something and have nothing else to do, it can be quite meditative to wile away the hours by going through your knots. I used to do that back when I was really into climbing…yeah a tad geeky but I knew my knots and could tie them without even looking.
The Figure Eight. Learn it…love it.