GUIDANCE NEEDED - Coding a 12 way rotary switch to control LEDs - HELP PLZ

Hey

I need to be able to code a 12 way rotary switch to turn on multiple LED's so that they can display chord shapes on the neck of a guitar. I have little experience with code and need so help. I have the Arduino Mega 2560 as recommended by some other people on this forum. Please could you leave a link to a video or simply just tell me how i could do this.

The switch needs to control 2 LEDs at a minimum and 4 at a max. Some LEDs are going to be used 4 times to show different chords. Wiring shouldn't be a problem because i know a electrician but the code is where i'm stuck.

Thanks for your help Sam