Focusing on the breath is a great place to start if you are new to meditation. The instruction is simple: pay attention to the breath.
When we do the breathing meditations we are not trying to change our way of breathing, rather we are just noticing how it is right now. How we are breathing gives us great feedback on our current state of mind. Noticing the qualities of our breathing—whether it is short or long, shallow or deep, and so on—can be really useful, as can becoming familiar with how we breathe normally and how the breath changes if we are anxious or angry.
It can be helpful to choose an area of the body in which you feel the breath most strongly—for example in the belly, the chest, around the nostrils or the lips, and always take your attention to this place when doing a breathing practice. If you find it difficult to locate a place, perhaps place a hand on your belly or on your chest, so you can feel the act of breathing.
Then, just notice the sensations of breathing—this is different from thinking about the breath. What we are doing here is feeling the expansion as the chest and the belly rise on the in-breath… and noticing the contraction as the chest and the belly fall on the out-breath. Remember, we are staying with the length of each in-breath, then the length of each outbreath. Whenever our mind wanders (which it will), gently escort it back to the breath and continue.
Experiment with tuning in to the breath at odd times during the day when you remember (no one else will know you are doing it.) If you would like to start practicing more formally, set aside five minutes to sit quietly somewhere and pay attention to the breath. Gradually extend the length of time, but remember the quality of attention is more important than the length of time you sit.
For more advice on breathing meditations, check out the Little Pocket Book of Mindfulness by Anna Black.