If it is the actual side bar contents that you want to edit, look for <div id="menu"> on index.php and/or go to yoursite/wordpressfolder/wp-admin/ to add or subtract links.
This page may help as well
http://codex.ww.wp.xz.cn/Main_Page
If it is the style that you are wanting to edit, take a look at this page:
http://www.tamba2.org.uk/wordpress/graphicalcss/
Hope that helps.
If there is no menu or sidebar section in the index.php of your theme, look for a sidebar.php file in the same theme directory.
It just occurred to me that this might be a question about 1.5… but I see you have beat me to the punch, Kafkaesqui.
