Here are 3 ways to do it:

The first way is to use the [wp_signon hook][1]

    $creds = array();    
    $creds['user_login'] = 'example';   
    $creds['user_password'] = 'plaintextpw';   
    $creds['remember'] = true;   
    $user = wp_signon( $creds, false );  
    if ( is_wp_error($user) ) {  
       echo $user->get_error_message();
    }

The second way, is to use jQuery `.val()` and add it to your login page

    $("#user_login").val('username');    
    $("#user_pass").val('password');

The last way is to edit your `wp-login.php` file (Note, this is not recommended although it would work).

[1]: http://codex.wordpress.org/Function_Reference/wp_signon