A pattern piece will usually indicate the hem both in the instructions and on the pattern piece itself. Almost every pattern I have done uses a standard 1¼" hem.
If you are making a more formal dress, skirt, or pair of pants, it's always a good idea to try the garment on before you hem while wearing the shoes you plan to wear with the outfit (for a bottom) or if you're a woman and this is a top, to wear the bra that you plan to wear with the garment (don't make me spell out why), and then determine where you would like your hemline to be. You can mark the hem and press it up, and then stand on a chair and have a friend or family member check to make sure that the hem is even all the way around. You may find that your hem is longer or shorter than the pattern's standard.
In either event, hemming requires a lot of ironing and pinning.
With the garment inside-out, you want to fold the raw/unhemmed edge up along your hemline. Start at a seam and match so that your seam allowances are even, and then work your way around the garment, using your seam gauge to make sure your hemline is neat. Wherever you encounter a seam, you want to get your seam allowances folded in on themselves for a nice even look. Press the fold, and then turn the raw edge under at a quarter of an inch so that the raw edge is enclosed on the inside of the hem. Press, and pin this in place.
Depending on the sort of material you work with, you will do different stitches for the hem. You'll want to stitch along the "top" of the hem to try to get the turned-under part in your stitches. If your fabric is very light--like a chiffon or a very light silk, you might forgo the machine and hand-stitch the hem using a whip-stitch (instructions to follow). Otherwise, depending on the material you're using, you will want to use either a straight- or zig-zag-stitch along the top of the hemline. I find that if I'm using a lightweight cotton, a straight stitch is sufficient. But if I'm using a heavier material, or one that resists ironing, a tight zig-zag stitch is a good idea.