If you dec now it's going to be too floppy at the brim which is where it needs to fit most. You'll have to start over.
Determine how many sts/inch you have, and what size it needs to be (a couple inches less than the head measurement) and use that number. If you want ribbing it should be an even number of sts for k1p1 rib, or divisible by 4 for k2p2 rib. 90 or 96 might be a good number, depending on how much too large it is now. When you decrease do k8, k2tog, or k10, k2tog around, knit a round plain, then k7 or 9, k2tog, knit one round. Repeat these 2 rounds with one less st between decs. When you get down to 3 or 4 sts betweeen dec, then dec on every round.