When you are going to study then make sure your fully committed to it. Avoid any aggressive thoughts, behavior and all distractions in your environment, like phone calls, TV or radio programs. Make sure you settle down to study in a quiet, comfortable and well-lit atmosphere. Take a few deep breaths; drink one or two glasses of water before you commence your studies. This will help you relax and feel motivated to study more. Ask yourself questions like why are we studying this subject, chapter or topic. What is so important or relevant in this topic, which we must know or discover for ourselves? What is the most challenging or difficult part in it and what information is new for us?
Give at least one hour to the subject at hand and always remember to take breaks of 2 mins. or so after every half hour of studying. This will enhance your retention of the subject. If you can study in a group of students then by all means do it. Interaction with fellow students will improve your grasp on the subject.
When studying always use pencil and paper to write down notes, meanings of terms, new or difficult words and sections which need explanation from your tutor. Instead of just reading quietly in your mind, read aloud slowly, talk to a fellow student to explain what you've just read or write down about what you have learnt or discovered recently. Discuss it with your parents, friends and teachers whenever possible. This alone will open more insights to the subject. When you find a section too difficult to understand properly then try to find out the dictionary meaning of the words which seem new or strange to you and discover exactly what the author meant by using these terms or words. Then try to put the meaning of those words in context of the sentence(s) and paragraph. The section will become much easier to follow now. Always write down meanings of new words, scientific terms etc. and recite them aloud a few times so that they are easily memorized.
Also try using your imagination to a great extent. Suppose, you are reading geography and come to a topic about volcanoes. Try to imagine in full detail, in gigantic proportions and in real life action about the volcano and how it may look like when active or dormant. This will put a mental image of the subject or topic firmly in your memory. If you haven't seen the object in real life or books or magazines then do try to find out online as the online articles often contain images, which will aid your learning.
Lastly, after studying a chapter or topic write down a brief summary about what you have learnt or understood. Always answer questions in writing when you find them. Show your homework and the difficult topics you have encountered to your teacher for clarification and correction. Also, revise every topic briefly but regularly at the end of your study session and early in the morning the following day so that it remains fresh in your mind. Systematic revision should be conducted once in every 24 hrs and you must revise the whole week's work done at school and home at least once. The same applies for revising the whole month's school and homework. This way you will never feel stressed about an upcoming test or exam since you will be better prepared than most students in your class.