Whenever you plan to add something to the edge of your garment, you should slip the first stitch of every row. You slip the stitches purlwise, i.e. yarn in front of your work, then continue knitting. This procedure turns the edge stitches slightly around and makes it easier to add a frame around a shawl for example.

Note: The first row after your casting on should be knit as usual, including the very first stitch. It needs a bit more support than the later rows.