First of all we must remember that we study that subject for some purpose. It can be to pass, to get a rank or to learn knowledge.
Next we must start developing a liking for the subject. The means to develop that liking can be anything. Association with a good friend, good teacher or support from family etc. can help in this.
Either select a subject that you are interested in or develop good interest in the subject you have selected to study.
Then study the subject regularly. Read again carefully and analytically if you do not understand it the first time.
If you are not able to concentrate, then relax yourself, take a walk around, talk with family members, use pen and paper to make brief notes, copy notes one more time etc. Keep yourself in a place without interrupts. Do not think of any thing else except the book, notes, exams, marks, rank , target, fame, future in your mind and in front of you.
Take some warm milk or tea/coffee/water. Do not eat too much. Do not see television while studying. Keep music at a very low volume, if at all music helps you concentrate. Reduce wavering of the mind by reducing avogations. Often good physical exercise, yoga, jogging, and fresh air breathing can help concentration. Often good scent, flower smell can open the mind. Sit in a place where there is a steady flow of fresh air. Sleep well and eat according to a discipline. Sip some water now and then.
Read, conceptualize, imagine, analyze, note down points, draw pictures appropriately, and try to be innovative.
Enjoy reading and learning. That can help you concentrate. Do not ridicule teachers and the subject and have respect for the knowledge. Then you will concentrate well.