Knits, stretch fabrics, and fabrics that fray easily are usually best finished with a serger. This makes them nice and neat and unlikely to unravel on you. Great, right? Sergers, however, are incredibly expensive (unless you buy a cheap one that breaks 2 months later) and not worth the investment unless you're looking to become a professional seamstress. So if you don't want to shell out money for a serger but you still want to finish your fabrics in a slightly-professional manner, you'll need to use a zigzag, or even a double zigzag, to give your seam finishes a decent look.
Before you begin finishing your seam, press it flat. You can do this by either pressing the seam allowances open or closed. Open results in a flatter seam when everything is finished, but closed looks more like a professional serged edged. It's really up to you. Remember that if you're ironing the seam allowances closed, you'll be stitching them together with your zigzag. If you're ironing them open, you'll be stitching them individually.
Either way, iron your seam before beginning. Once this is done, stitch a wide or medium-wide zigzag stitch near the raw edge of the seam allowance. If you're not sure what size stitch to use, practice a bit on a scrap piece of fabric. Change your stitch widths a few times and see what works best. In general, you'll use a wider stitch for heavyweight fabrics and a narrow stitch for lightweight fabrics, but this is only a guide. Use what works. You can trim your seam allowances if you like, but be careful not to clip the stitching or you'll have to start all over again.
The zigzag seam finish is probably the most common finish for most of us (including me because I have a love/hate relationship with my serger). It tends to be the go-to seam finish, but don't be afraid to experiment with others.