Req: basic info on setting up subfolder and ftp

I am new to working with the web and would like some very basic information.
I’ve got a public website set up. I would like to set up a subfolder, that is not publicly visible, which only I or my colleagues can access with a password. I would like to be able to post data to that subfolder, presumably using FTP.
Would somebody be kind enough to provide me with step-by-step, very basic instructions on how to do this?
Many thanks.