You should try to focus on both nostrils, I think focusing on one would be too much of a challenge.
Breathe naturally, don't control or manipulate your breath in any way. Breathe as you normally would, but simply focus your attention on the nostrils, pay attention to how it feels in that area, hot? cold? rough? smooth? Just be aware of how it feels around that area as you breathe normally, don't worry about controlling your breath in any way.
For me, I focus on my nostrils because that is the area in which I feel the breath more strongly. If you can't feel the breath so well in the nostrils, where do you feel it the strongest? In the throat? the chest? the abdomen? wherever you find it easiest to focus, you can fix your attention there.
Yes you can develop states of deep concentration, however don't think about this when you sit down to meditate. Thinking about achieving this will take your mind out of the present moment, all you should do is watch the breath in the moment. Don't think about what will happen in the next moment.
I hope that helps
"For me, it is far better to grasp the Universe as it really is than to persist in delusion, however satisfying and reassuring."