I only want to modify one tpl.php for a theme, do I need to copy the base theme? I only need to have one tpl.php added to a theme,but the theme I am using is not my own.  If I still want to have this theme as my "base" and then create my own theme with the tpl.php using a theme suggestion I want, how do I get this to work.

I have tried setting the base theme as the default and then creating my own theme with the tpl.php I want and enabling it, but it isn't working. If this is the correct way to do this, what is the minumum needed in the .info file and the folder for this to work?
The minimalist amount you need for a subtheme info file are the 2 required keys, name &amp; core, plus the declared base theme key, base theme. 

If you a create info file with those keys, copy over the tpl.php (e.g. page.tpl.php) from the base theme, modify it, flush caches (to rebuild the theme registry), and use the Enable and set default from the theme administration page, you should have a working subtheme.

See the d.o. docs on creating a subtheme for more info.