The second-most common stitch in the knitting world is the purl stitch, which is sort of the opposite of a knit. Once you know how to knit and purl, you can make all sorts of patterns by combining the two stitches.
Lay the yarn loosely underneath the loop that is on the peg. Insert the hook under that loop, and scoop up the working yarn, thus creating a new loop.
Take the existing loop off of the peg, and replace it with the loop that is pulled through.