Newbie Child Theme Basic Questions

Need help understanding what must be transferred to a Child Theme. I have modified my Twenty-Twelve theme only in two ways:

1. Using the Appearance-Customize section – changing simple things like background image, header image, sidebar hierarchy, etc.

2. Added Function PHP code to convert my old BlogSpot to WordPress

Now I want to install a Child Theme. Please advise what I need to toss into the Child Theme. The basic Appearance changes? The Function PHP?

Thanks much!