Having a tech issue… anyone know about arduino atmega328?

Having issues with knowing what and where to add code. First time building something circuit wise. I know we got some very tech savvy people here. Could really use some help understanding this stuff.

Here's a start:
Arduino 101 Fundamentals : 7 Steps (with Pictures) - Instructables

I think I must have gave the appearance of knowing what I’m talking about. I truly have never done code of any kind. So I may have got in over my head here. It’s a prebuilt system and I’m just stumbling over it. Built the damn thing with ease… even boxed it all in a dewalt bit box. Just don’t know what I’m doing with the whole code thing.