I've decided to join the Google AdSense program. Now you can see advertisement blocks at the sidebar. Hope they are relevant, and that it may give me revenue.
The changes were quite trivial. In the folder "./wp-content/themes/default" I created the file "adsense.php" and in the file "sidebar.php" I added near the end:
<?php include (TEMPLATEPATH . '/adsense.php'); ?>
It is enough to show ad blocks on all pages with a sidebar. A personal page for a post doesn't have a sidebar, so I also changed the template "single.php".