To begin your cable row, purl the first three stitches, just like you did on the first right-side row.
Now you will be making the cable twist in the next six stitches. To form the twist, three stitches are held on the cable needle while you knit the three stitches that had been following them and then knit the three stitches off the cable needle.
It sounds confusing, but it's really easy. To begin, slip the first three stitches you would knit on this row (following the three purls) onto a cable needle. These are slipped "purlwise," or in the same manner, from back to front, that you would make a purl stitch.