Sweaters can be knit in the round or flat.
This is the stitch pattern I found for Fisherman's Rib:
Round 1: Knit 1, Purl 1; repeat around.
Round 2: Knit 1 into stitch below, Purl 1; repeat around.
Round 3: Knit 1, Purl 1 into stitch below; repeat around.
Repeat rounds 2 & 3 to desired length, bind off.
So, begin with Round 1, you need to convert Round 2 to a right side row which would be "purl 1 stitch into stitch below, Knit 1; repeat around." Round 3 would be as written, and then you would repeat your converted Round 2 and Round 3 as written to desired length.