A couple years ago, I posted instructions on how to make a knitted picot hem. It's a handy trick. In crochet, if you want a folded hem, the easiest thing to do is work the row where you want the crease to be in the back loop only.
Here's a folded hem on a finished hat. The right side is facing out on both the body and the hem, a row of knurling marks the fold line at the bottom, and the hem was tacked in place by working some stitches through both layers of the fabric.
A more attractive method is to do a little knurling. Knurling is where you work a row of crab stitch in the front loop only and then work the next row of the fabric in the remaining loop. It's usually used to create a raised line in Aran-style crochet. While crab stitch is traditional, I prefer light crab stitch, as it is usually a better match for my overall gauge.
If you want a hem where one side folds up on the outside, like on the brim of a hat, work a row of crab or light crab stitch in the front loop only, then turn your work if you are working in the round, and work your next row in the remaining loop, which is now the front loop (facing you). By turning your work, the right side will be facing out both on the folded up cuff or bring and on the body of the piece.
If you are working in rows, cut your yarn after the crab stitch row, turn your work, and reattach the yarn on the end where you began the crab stitch row, working the next row in the remaining loop. Now the right side below the crab stitch will be on one side, and right side above the crab stitch row will be on the other side.
The bottom part of the picture shows the hem, which will fold up on the outside. The right side of it is facing you. The middle part is the row of knurling along the fold line, and the double crochets on the top are the first row of the body of the hat. That row has the wrong side facing, because the hem will be folded up in front of the right side, meaning the right side of both the hem and the body will be facing out when the piece is done.
If the hem folds inwards and will not be visible on the finished piece, don't bother turning your work after the crab stitch row, because it doesn't matter. You also shouldn't turn your work if you want the right side of the fabric visible on the inside, such as if it were the interior layer of a bag or a double layer, reversible hat.
If you don't want to knurl or want a more elaborate decorative edge, you can also work any kind of pretty one-row pattern stitch in the front loop, and then carry on with the body of your project in the remaining loop. Just make sure to arrange everything so that the right side of that decorative edge will face out on the finished piece.
If you worked the hem first, then after you have worked the same length after the knurled row, you can work some of your stitches through both the current row and the underside of your foundation row to tack the hem in place. If you work the hem after the knurled row, make your final row (sc1, ch1) across or around. Then make some of your chains slip stitches worked into the corresponding stitches in the body of the work.