Daniel:

Here's the plugin you want: WP Limit Posts Automatically plugin for Wordpress - more tag alternative, cut descriptions, wordpress plugin | jenst.se

You can change the sidebars by using a text widget and entering html for your ads. For google, you'll get the code directly from your adsense account.

Changing the header would be a matter of uploading your new header to an images folder on your site. I'm not familiar with this particular theme so can't tell you the specifics without seeing your header file. Someone else will hopefully have some insight.

PS: Here's a thread with good advice on the best plugins for WordPress.
http://www.warriorforum.com/main-int...s-plugins.html

Go into the Theme Editor and then make changes to the "sidebar.php" file. I'd really just start putting in a few letters/numbers and then saving it to see where they pop up for you so you get a better understanding of how the system works.