The best time to start preparation is at the beginning of 11th class. Students are fresh after the break after boards and a lot more can be learnt. Students who start late often end up leaving some key topics of 11th class JEE syllabus. Also, if you start your preparation in 12th class then the amount of hard work is increased manifold due to additional pressure of 12th boards, so chances of getting a high rank in JEE are decreased. There is a big transition from the mugging up study of 10th class to more understanding based study of 11th class. So you shouldn't get bogged down by seeing the level of complexity in JEE exam.
The first two or three months should be spent in revising your basic concepts (topics like algebra, mecanics) which had been taught in 9th class. Concepts in topics like algebra, mechanics and stoichiometry should be studied in great detail as they also form the basis for other topics and are very scoring. There should be no doubt in your mind about these concepts. For these, develop the habit of inquisition. Clear all your doubts so that mentally you feel confident tackling any concept. A key in this area is problem-solving. Regular problem solving not only strengthens the concepts, but also increases your mental sharpness which is must for JEE.Initially problems of low level should be tried and only when you have mastered them, you should go on to solving intricate JEE type problems. You should have systematic plan for the preparation of JEE. Regular hours should be allotted to mastering concepts and problem-solving. It is not necessary to memorise the lessons and formulae. Understand the derivations of formulae and apply the formulae to solving problems. Slowly these formulae will settle down in your brain and you will be able to recall them under any situation. What is required is a clear understanding and an easy familiarity with the concepts and laws. Maximum attention should be paid to solving problems.Try to be analytical rather than being intutive. Remember that hardwork pays! You should first take up a problem and try to solve it yourself. You should understand what is given and what is required, recall what principles, laws, equations are involved. Don't hurry to see the solution as I doesn't help you to learn. Only when you've pondered over it over a long time and unable to see any light, should you see the solutions. You would be insulting yourself and seriously hurting your chances of success if you skip these problems or make half-hearted attempts in solving them. An other very important aspect of your JEE preparation will be the coaching institute, whose importance and selection procedure will be delved into in the next section.