How do I go about selling gold?

I’ve got some gold jewelry that I have never and will never wear, and I was thinking of selling it while gold is so ridiculously valued. If I have the weight of the piece, and the 18K stamp, how do I calculate how much actual gold is in the piece? Who buys gold (besides the oh-so-sketchy mail-in ads)? Is there a standard fee? How do I ensure that I don’t get ripped off?