Some people get to one stitch before the end of the row, then swap the last stitch for the first stitch on the circular. I don’t bother. If yu’ve got a hat-sized circular or magic loop, you get to the end of the row and just keep going. There are some fancy ways to hide the beginning and end of stripes and so forth, but if you’re not striping don’t even worry about it.
Remember you’re working in the round, so knitting every stitch will give you a stockinette stitch tube. If you don’t put ribbing on the bottom, it’ll roll, but that’s not a bad thing because it makes the hat stay on almost as well as ribbing. If you want the piece to look like garter stitch, you have to purl every other row.
I bet you’re going to enjoy making hats. They’re one of my favorite things to knit, right up there with fingerless gloves.