ahhh, this is a good place to start experiencing how ZC works.

first:
Use the admin/tools/developerstoolkit - go to the bottom of that page, if you put in tpl_footer.php into the search box and change the dropdown menu in the middle to look for "all" - it will show you all the places that have a file with that name - you can also search for words that you might want to edit/change. Like "final step" changed to "almost done" etc.

second:
When you see a reference like that: YOUR_TEMPLATE/common/tpl_footer.php
the capitalized YOUR_TEMPLATE is what you would rename your template if you were using the override system - which you need to do soon or you will have real problems during upgrades.

Since you are using the 'classic' template you're going to find this file in either the includes/templates/classic/common or includes/templates/template_default/common; but eventually you should find it in includes/templates/template_ scrapbooks/common

glad I can help, it took me awhile to wade through this stuff too