How do I lock (password protect) files or folders ?

There are many ways to protect  (password protect) or lock your files and folders for selective use or selective access. Below are some of the simplest solutions to address your problems.

AxCrypt: An excellent FREE encryption utility that enables users to encrypt all files within a folder and not allow those files to be viewed unless a (password) is known. (Yes its FREE!)

WinCry: A freeware utility that enables your files to be encrypted, secure deletion, as well as other helpful methods of protecting your files.(FREEWARE)

Folder Guard: A freeware utility that enables your files to be encrypted, secure deletion, as well as other helpful methods of protecting your files (PAID VERSION)

Hope this helps.