How can I learn to concentrate better?

Answer: Practise meditation for half an hour a day

Meditate To Concentrate
ScienceDaily (2007-06-26) — Researchers say that practicing even small doses of daily meditation may improve focus and performance. Even for those new to the practice, meditation enhanced performance and the ability to focus attention. Performance-based measures of cognitive function demonstrated improvements in a matter of weeks. … > read full article

and this:

Meditation helps increase attention span
ScienceDaily (2010-07-16) — It’s nearly impossible to pay attention to one thing for a long time. A new study looks at whether Buddhist meditation can improve a person’s ability to be attentive and finds that meditation training helps people do better at focusing for a long time on a task that requires them to distinguish small differences between things they see. … > read full article

and this:

Meditation May Fine-tune Control Over Attention
ScienceDaily (2007-05-10) — Everyday experience and psychology research both indicate that paying close attention to one thing can keep you from noticing something else. However, a new study suggests that attention does not have a fixed capacity — and that it can be improved by directed mental training, such as meditation. … > read full article

Read more:

What Can 11 Hours of Meditation Training Do? It can Rewire Your Brain
Meditation trains our brains to regulate emotions and behaviors