Once you have cast on, you need to distribute the stitches onto three of your four needles. To do this, slide your stitches to the end of the needle, beginning with the first cast on stitch, and slip the stitches one at a time onto another needle.
You'll want to have seven stitches each on two needles and six stitches on the other needle.
Once you have your stitches distributed, join the work in the round. I do this by slipping the first cast on stitch onto the needle with the last cast on stitch and then slipping the last stitch over the first stitch and onto the last needle.
Alternatively you can cast on an extra stitch, slip the last cast on stitch onto the needle with the first cast on stitch and knit them together. Or use whatever method you like.
At this point you can slip a stitch holder onto the needle to mark the end of the row, or just note where the tail from your cast on is; this is the end of the row.