Knitting-how to keep stockinette stitch from curling?

you should definitely do an inch of ribbing (k1p1) first to keep it straight and elastic around the forehead. if you are knitting the hat in the round (instead of flat and then sewing up the sides) you dont need to learn stockinette stitch. knitting in the round automatically makes it stockinette, so you would just have to knit the whole thing. its also nice cause there is no messy seam where you had to sew the hat up. good luck!