Divide your portion of studying in small parts.
Make your study sessions small.. Maybe as small as 15 mins. Set precise targets for achieving the target in 15 minutes. Take 15-20 minutes break during which you can do some light exercise or recreate
Revise whatever you have studied at the end of day and before beginning next day.
If your inability to concentrate is a recent happening and you have some symptoms apart from this, consult a psychiatrist to rule out depression or attention deficit disorder.
Attention span and concentration differs from person to person. these are some general tips to study efficiently:
Make notes of things you would like to remember.
Revise the topic you studied previously before you start a new topic .
Take regular breaks every forty five minutes . Do not use mobiles, computers during this time.
Get good restful sleep.
Get enough physical exercise during the day.