bring the mind back, again and again. What do you notice about the breath? Is it quiet? Is one nostril a little stuffy? Do you feel the coolness of the breath as it moves into the nose, and the warmer quality as you move the breath out? Observe all this without any judgement. If you don't have access to a meditation teacher, this is a good place to start.
When I was a beginning yogini, after practicing with the Integral Yoga Center in Danbury Connecticut, I had the opportunity to go to a 10 day silent retreat. I'd been practicing seriously for about a year and a half, and had already met Swami Satchidananda a few times, and decided that he should be my Guru. I really wanted to go to this retreat, so I came off the road (I was in an all-female band, Ira Gobu at the time) and signed up. Towards the end of the retreat, the staff were offering an initiation with the Swami, and I definitely wanted to do that! We were encouraged to fast for most of the day, and get cleaned up (we were all hippie types) and wear white clothes, a symbol of purity. I was really excited about going deeper into yoga.
There were prayers, a devotional service called a puja, and then each one of us--over 50 people--went up to Swamiji and told him our name. He gave each one of us a mantra--a short Sanskrit sound formula--for us to use in meditation.
We were given a little booklet about how to meditate using the mantra. We each were given a mala to use as well--a set of rosary beads used for meditation. I already knew many of the prayers and breathing practices that were recommended to help calm the mind for meditation from being at the retreat, so I felt I was ready to go with my new mantra, my mala, and lo and behold, my new boyfriend whom I met as the retreat was ending!
I'm sharing all this to let you know that focusing on your breath, which is where I started this discussion about concentration, is not the only way to go. My own personal practice is to concentrate on my mantra. There is a subtle power in the sacred mantra, and since that was what my Guru was telling me to meditate on, I didn't argue.
There are many other ways to train the mind to concentrate which you can read about in my book, Big Yoga For Less Stress. But, pick something that isn't too difficult. Pick something that is uplifting, or as Swamiji said "Anything that delights the mind". Stick to it, and have the faith in your practice that you are keeping the brain healthy, and bringing more peace into the world. Your peaceful presence is badly needed in the world today!