Who is the hat for.. baby, child or adult? Is it a plain stockinette stitch hat or does it have a pattern to it? Are you knitting in the round or flat? With simple stockinette you can usually just cast on less stitches, but if there is any pattern or ribbing it often needs to be different.
We can help you better if we know the answers to these questions. And if you have a link to the pattern please add that, too. The more info you can give the better it is.