To train your mind to think only one thing at a time, you require meditation
. It's a little difficult so you can do chanting for one min to 10 mins gradually increasing the time. Or deep breathing exercises will help.
Some people's brain can do two things at a time. Just check if your brain has tat capacity then you can put a soft music in the background and then study. If you feel it's difficult to study then concentrate on breathing exercises so you can train your brain.