How do i knit a circle?

i searched the website and lots of links but i'm still clueless on how to knit the top of a ski cap i've been making. i need to knit a circle to attach to the band i've already knit as the base of the hat, but i have no idea how to do it or if i need round needles to make it work. can somebody please give me some simple, easy-to-read instructions for a beginner on how to knit a round piece with straight needles? oh, and not a hollow circle of course but a coaster-style type deal. i need help!