In order to have even decreases, you have to do a little math
If the number of stitches is divisible by 10, for example, you could start with k8, k2tog all the way around; then knit one round plain.
For the next decrease, you'd k7, k2tog followed by one plain round.
If you prefer a slower decrease (for a more pointed hat) you'd start with a higher number pf stitches between decreases. For a more rapid decrease (for a rounder top) you'd start with more decreases per round.
If you can't find a number that is easily divisible for your number of stitches, decrease to a more even amount on the round before you start your decreases.
Keep doing thus until you've got about 5-10 or so stitches left. Cut your yarn about 18" long, thread it on a blunt-tipped needle and put the yarn through the stitches on the needle. Take out the needles, pull tight like a drawstring. Put the sewing needle down through the hole and weave it in around the inside of the hat.