Dear jeffS-2,
for setting up a child theme you may want to review the following wordpress documentation
http://codex.wordpress.org/Child_Themes
http://codex.wordpress.org/Template_Hierarchy
if you want to change the PHP code for the header template file, you can include a header.php in your theme's directory.
Your child theme can overwrite any file in the parent theme this way.
Please let me know if you are satisfied with my answer and if I can help you with any other questions you might have.
Regards,
Ana