How to rewrite URL and get the values?

The simplest and quickest solution would be to use the add_rewrite_endpoint function.

<?php add_rewrite_endpoint( $name, $places ); ?>

So in your case it would be something like this:

add_rewrite_endpoint( 'user', EP_PERMALINK | EP_PAGES );

An user URL would look like this:

http://www.example.com/my-page/user/john

To access the variable:

$user = get_query_var( 'user', false );