The brain is like a muscle. If you want to get better at focusing, practice. Meditation is a great way to do so. Count your breaths 1 through 10 (inhale, one, exhale, two, inhale three....) and then start over when you get to 10. Every time you realize you have been distracted, bring your attention back to the breath. It's hard to do consistently without getting distracted, try it, you'll see, but it will get easier the more you do it, and this kind of brain training has been shown to have a demonstrable effect on our ability to focus. This is the best advice I can give you.
The science shows us there are more extreme things you might do - like putting on noise cancelling headphones, or sitting in a quiet room when you are reading. There is a young neuroscientist at NYU named Michael Halassa who works with ADHD patients. He has discovered some interesting things about the way the brain's attention circuits work. The prefrontal cortex has sometimes been compared to a "track switcher" - it's this part of the brain that controls what sensory information gets priority to travel to the areas of the brain that register the things we are consciously aware of.
If you are chatting with a guy named John at a cocktail party and are actively trying to focus on him, for instance, the prefrontal cortex will send a message to a part of the brain called the Thalamus. The Thalamus will make it easier for sounds emanating from John to reach your conscious awareness, and may even provide resistance to distracting sounds.
Halassa, however, has found that some patients with ADHD have deficits in their brains that make it harder for them to drown out irrelevant sensory noise. They might have no problem paying attention to John in a dimly lit quiet rooms. But they can't focus when surrounded by other sensory stimuli and they don't know why. Others with ADHD have problems with their executive functions. The track switching director doesn't work so well. For them, this solution might not work as well. But for the rest of us, even those of us without ADHD, it's useful to know.
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