how to copy php from parent theme into child theme

I’m so sorry if this is too basic of a question for this forum… but I cannot find the answer to this (probably simple) dilemna of mine…:
I have previously created a child theme of TwentyTen, and have edited the style.css to create a basic (my very first ever) website. Now that I need to make more edits etc., I realize that I don’t have any of the other php files in my child theme’s editor, only the CSS stylesheet (no header.php file or other).
Where do I find that, and how and where do I copy it from and to?? Seriously, I don’t even remember how I managed this child theme in the first place… it truly is my first time doing any of this, huge learning curve…
I would be so appreciative of any of your help talking me through this. Thank you!!