How to start with post number x?

I solved it myself. What I did, is adding this:

                    $loop = new WP_Query(array(
                        'offset' => 20,   
                    ));

And then changing:

if (have_posts()) : while (have_posts()) : the_post();

into:

if ($loop->have_posts() ) : while ( $loop->have_posts() ) : $loop->the_post();