Usually when you knit with circular needles, you join the stitches so you can work in a circle and end up with a long tube, like the bottom of a sweater.
Because of the number of stitches on a blanket, you use a circular needle just for the length needed to accommodate the stitches
Just work back and forth on the needle as you would with straights, as if each point was a separate needle.
Cast on is just cast on. It's not the first row.