It depends on how many sts you have as to how you begin. You find two numbers that divide into your stitch number and use them as the basis for your decreases. Hats usually work well when there's 8 (or 7 or 9) decreases around.
Say you have 80, if you do k8, k2tog that makes 8 decs around. Then you knit a plain row and the following row you would k7, k2tog. Still keeps the 8 decs, but you've got to go down one stitch between the k2togs. Dec the same way every other round, one st less between decs, until you're down to about 3 sts between them, then dec on every round until you have about 16 sts left. Then you cut a tail and thread it through the remaining stitches like a drawstring.
You can still keep the rib pattern going, but your ribs will shrink down to 2x1 then 1x1 and by then it won't matter if they're ribbed or not.