Problem setting permissions for various files and subdirectorys

[José Paulo Matafome Oleiro]

Hi there again guys :)
Today I've got a new problem since I'm a newbie at GNU/Linux, and
working with the command line right now to me is very complicated, since
I hadn't read a good tutorial for introducing me to the shell commands
and everything available. I know the command line is the true power of
GNU/Linux. If you know of one please can point me. 
Now talking about my problem.
I've got thousands of files in my desktop pc, where I do a backup for
files. I wan't to set read and write permissions to everybody for all
files under the main directory, how I can do this in the command line?

I know that I need to use chmod but I never used it and I don't know how
to use :(
Sorry about my lazy english.
Thanks a lot for your help.
-- 
José Paulo Matafome Oleiro <matafomeoleiro at gmail.com>