For all users to have read and write access, that would be 0777 which is a bit dangerous, especially if you are running a webserver.
Like @unwind said:
chmod -R 0777 /mydirectory Will allow all users read and write access to all files and folders within that directory
Depending on your purpose, you may want to read about sticky bits, which allow all users to create new files, but not to delete or edit other files in a directory:
chmod +t /mydirectory
Also, in case you didn't know man chmod will bring up the manual page for the chmod command, which you can search for the text "recursive" by typing /recursive