To help you concentrate during Nitnem you can put a Nitnem tape on and follow along with it. But sometimes, it is quite hard to focus, since our mind can get very racy. Therefore, try to do path in a quiet room, with minimum amount of disturbance. Put a tape on if that works for you and try to feel the love of the Guru (which will come gradually). However, if you are still not able to concentrate, don't lose hope -just keep doing it and slowly but surely, you will start feeling the focus and love and then the challenge that many face is to maintain that energy on an everyday level.