There are two commonly-used methods of casting on. The thumb method uses one needle and tends to create a less noticeable edge to your knitting. The two needle cast-on creates a firmer, less elastic, but stronger edge and is suitable for delicate yarns or garments which will be subjected to very hard wear.
Before you can start casting on you need to make a loop. Take the yarn from the centre of the ball as this stops the yarn rolling about when you are knitting. (For the thumb method you will need to leave a length of yarn roughly 6 times as long as the width of your intended piece of knitting).
Fig 1 Hold the loose end of the yarn in your left hand and the needle in your right hand. Form a loop with the left thumb by placing the thumb under the yarn from behind and then move the thumb upwards to form a loop.
Fig 2 Place the loop on the needle and keeping the left thumb in place.
Fig 3 use the yarn from the ball in your right hand, to wind yarn round needle clockwise.
Lift loop over needle and remove thumb. Pull left-hand yarn tight.
Continue until you have sufficient stitches.
two needle method
Fig 6 Make a loop as before and place on left-hand needle.
Fig 7 Pull the right-hand needle through the loop from the front. Wind the yarn round needle clockwise.
Fig 8 Pull loop through stitch, and place loop on needle.
Now you have stitches.
To create a neat edge the remaining stitches are cast on slightly differently. Put the right-hand needle through between the 1st and 2nd stitch, then pull loop through and place on left-hand needle as before. Continue until you have sufficient stitches.
Now that you have cast on your stitches, you need to learn how to knit a stitch!