Some hats you knit in the round - either on double pointed needles or on a circular needle. There are great videos in the videos link above on circular knitting. The biggest difference from knitting flat, is that instead of knitting your last cast on stitch first, you knit the first cast on stitch first.
Other hats, you knit "flat" then seam (sew) up the back after you're done knitting.
Knitting homeschooling mom to 3 kidlets
Bayerische Socks by Eunny Jang in Knit Picks Risata (for ME!)
in queue: Swirl glittens; lacy raglan for me; new cabled sweater for dh