Welcome melany berry!
The cast on is not usually considered a row; but with long-tail cast-on, it creates an already knitted row in addition to the most basic cast-on edge. If you are working in garter stitch (all knitted rows), it's fine to begin by knitting. If you're working in stockinette stitch (knit one row, purl one row, alternating), then for the least noticable cast-on edge, you'd begin with a purl row. You could begin with a knit row, it would look fine too, just more noticable or decorative.