In order to start knitting, you must first get your yarn on the needle. To do this, you make little loops, or stitches, around one of the needles. Your knitting will be as wide as your cast on row if you do not apply any shaping, such as increasing or decreasing. This method of casting on is useful because it is nice and sturdy, as well as very easy to begin knitting into, as it is essentially your first knit row.
Step 1: Make a Slipknot
This is simple enough. What's tricky is leaving the right amount of yarn for your tail.
I plan to cast on 15 stitches, and I'm using pretty big (10 1/2 US) needles, so I'm going to leave two feet just to be safe. This is probably too much, but get used to overestimating, as it will save much time and frustration in the long run if you end up redoing it.
Make a loop around your hand and cross it so that you can see the end running to the ball of yarn through the loop. Pick up the "bar" inside the "circle" and tighten slightly by pulling the ball end of the yarn.
Place the loop on your needle so that the free end of yarn is closer to you.
You should be holding your needle like a bike handle bar. While maneuvering the needle may not be too comfortable at first, you should be fine just holding it this way.