how to lock folder ?

you can change the permissions of the files/folder to restrict other users and groups from having access to the contents.

You can read here for more about permissions:

Maybe GUI would be easier. Right click folder and select properties, now pick tab called permissions. Adjust permissions how you like it for all three groups eg read only. Optionally apply permissions to enclosed files.
To learn about chmod in terminal execute man chmod.
For example chmod 0777 path is allow everything and everybody. Try some combinations like chmod 522 path
Use ls -l path to see what is outcome.