Where the hell do I find & open configuration h files?

Ed.

Get Marlin from [github.com], use the 'clone or download' button to get it as a zip file. Extract it to it's own directory.
Get the Arduino IDE from [www.arduino.cc] and install it.
Start the Arduino IDE, then File->Open, browse to where you extracted the zip file, in the Marlin directory open Marlin.ino then click on the confirguration.h tab.

If needed, see [www.youtube.com] for videos on how to set up Marlin.

Or, if you're using Repetier Firmware, the online config tool will handle all the settings in configuration.h for you, and provide a download all button at the end of the process. You still need to install the Arduino IDE to be able to compile the downloaded code and upload it to the hardware.

On newer 32 bit hardware configuration is handled by editing a text file without the need to recompile the firmware and upload it. Sadly I don't have newer 32 bit hardware, so I can't claim to know the details!