I only want to modify one tpl.php for a theme, do I need to copy the base theme?
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.