How do I create new unix executable (.sh?) files for the terminal to use as commands?

I'd like to make some files to put in my /usr/bin folder (Mac OS) and be able to run them by typing the name to the terminal, like the commands gcc, cd, or vim (those are all in that folder). When I open such a file as text, it appears encrypted, so I'm not sure how to create one or what extension to use. Thank you.