Rear brakes worn on one side, how do I recognize the reason as layman?

Hello everyone, I am actually for the first time with a question in the Biker Forum, because I have only had the appearance for 10 months and at the same time with a lively question. The withdrawal brake with my CBR 500F PC20 is almost completely down on the inside and since I am currently close to the cash register I would like to fix it myself. Problem > I have to recognize the cause. What is there for which and how do I find out where exactly the trigger is now? Unfortunately, I am not so technical gifted thanks and DLzG