For those of us who just don't do a lot of Terminal-ly things and don't want to figure out how to change directories and stuff, the easiest way I've found to password-protect a zip file is just using Keka, a free Mac app at
http://www.kekaosx.com/en/.
All I had to do was open the app, type in my password that I want on my .zip file and just drag and drop the .zip file onto the app in my dock.
Voila! The encrypted .zip file appears in the same folder as the un-encrypted .zip file.