Create a child theme:
1. Using an FTP client (I recommend using FileZilla) create a directory under wp-content/themes/ and name it anything you like (e.g. LeonSterlingTheme)
1. Create a text file named style.css with the following content (between the ==== lines, but not including them):
===========================================
/*
Theme Name: Leon Sterling's Theme
Description: Twenty Eleven Child Theme
Author: Leon Sterling
Template: twentyeleven
@import url("../twentyeleven/style.css");
// Theme customization starts below here
===========================================
1. Put (using FTP client) the above style.css file into the themes directory you created.
1. Download the wp-content/themes/twentyeleven/footer.php file and delete the following text from that file with a text editor (e.g. Notepad or TextEdit) and save it.
===========================================
<div id="site-generator">
<?php do_action( 'twentyeleven_credits' ); ?>
<a href="<?php echo esc_url( _( 'http://wordpress.org/', 'twentyeleven' ) ); ?>" title="<?php esc_attr_e( 'Semantic Personal Publishing Platform', 'twentyeleven' ); ?>"><?php printf( _( 'Proudly powered by %s', 'twentyeleven' ), 'WordPress' ); ?></a>
</div>
===========================================
Upload the newly edited footer.php file to your theme’s folder.
1. From your WP Dashboard’s Appearance->Themes page, activate your child theme.
This will create a new child theme that you can use for any other modifications you would want to make to the Twenty Eleven theme.
For more details, see: http://codex.wordpress.org/Child_Themes