It takes a bit of getting used to, but if you've learned HTML and CSS or anything like that, it's not impossible. The thing to remember is the PHP tags are not like HTML and CSS - they don't change how something looks, they change what is displayed there.
Moshu said: Find this line:
<?php wp_list_pages('title_li=<h2>Pages</h2>' ); ?>
and replace Pages with whatever you want.
You said: I finally found the second sidebar.php, it contained the wp_list_pages command. However, after studying it, I have no idea "what pages I should use to replace what pages."
If I understand what Moshu said, he meant you can replace the word "Pages" in
<?php wp_list_pages('title_li=<h2>Pages</h2>' ); ?> with another heading, like "About This Site".
That will change the word "Pages" to "About this site" (or whatever you choose to type in between
</h2> in that string, and it will display a list of the pages you have created in your WordPress blog underneath it.
If you want something else displayed instead of those pages, you would then need to put a different php tag in that line. For example, if you want the blog's description to display underneath the word "Pages" or "About this site", you would need the following:
<?php bloginfo('description'); ?>
and you would put that AFTER the above line of code that contains "Pages" or "About this site".
I find this page enormously helpful: http://codex.wordpress.org/Stepping_Into_Template_Tags
Hope this helps!