Are you referring to seaming up the side of the hat?
If so, the best seam to make is called the "mattress stitch". Amy made a video on how to do it on this webpage:
Scroll down to the section on "Finishing", and you'll see a few different seaming options. Just click the button to see the videos for each of them.
You'll need to use the same yarn you used for your hat, so it will match.