Starting retropie for the first time.
I used a keyboard for this but other controllers including but not limited to would be (Xbox 360 controller, Xbox one, ps3, ps4 and Super Nintendo controller.)
First you will prompted to begin with the keyboard bindings.
You will want to enter them in using whichever controller you are using, A will be your main select button and B will be your main back button.
After entering the controls hit whichever key you binded to A on the Ok button on the end.
Retropie comes with basic games installed such as DOOM in the next journal I will show how to get more emulators and games.