.htaccess with www and without www

[xxdex]

how to change a .htaccess to working
with or without a www.
i have a page with a begin at start "www".. and i want to force web users.. to go to a page without a www. at begin.. i know that is a 301 forwarding.. but i dont know how to use it with a get simple:
this is a original file:

PHP Code:

RewriteEngine on

# Usually it RewriteBase is just '/', but 
# replace it with your subdirectory path
RewriteBase /

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d

RewriteRule /?([A-Za-z0-9_-]+)/?$ index.php?id=$1 [QSA,L] 


i'm was try with a:

PHP Code:

RewriteEngine On
RewriteCond %{HTTP_HOST} ^mydomain.com$ [NC]
RewriteRule ^(.*)$ http://www.mydomain.com/$1 [R=301,L] 


but i dont know how to use it with a .htacces file in get simple. can some one help.. and help analogously how to use it from a www.mydomain.com to http://mydomain.com