IIT JEE examination is conducted in two phases to select candidates for admission into Ist Year B Tech/BE/b Arch in the national Premier Institutions like NIT, IIIT,National School of Mine, ISER & IITs. The Phase I is JEE (Mains) examination which is a qualifying examination for the JEE (Advanced) for admissions in IITs under Phase II. The ranks would be devised based on the performance in the Advanced examination for admission into IITs while the ranks would be given separately for those who qualified in the JEE (Mains) examination for admission into NITs/IIITs etc., The JEE (Mains) examination is conducted by CBSE while the Advanced examination is conducted by on the designated IIT to do so for the year.
For the preparation part, our friends have suggested well above. Apart from that, it is ideal to have a better coaching for it. There is a wide range competition and you should focus on comprehensive continuous evaluation system of the subjects Physics, maths and Chemistry. Referral books like Advanced mathematics by A Das Gupta, Various Chemistry books written by R K Jain and Arihant series for Physics would be helpful. You have to analyse the pattern of examination for the last 5 or 7 years for the examinations and try to get easy tips for fast solutions from your tutor or college teachers. The coaching institutes would conduct weekly tests and don't miss them as this would help you to increase your quickness and analyse your knowledge and let you know how fast you are. If you can afford, purchase monthly magazines like Physic for all, Chemistry for all etc.,or other JEE related monthly magazines regularly which would give a better idea for you. It is not that much difficult if you put your efforts very seriously and work hard for three to four hours regularly and sure you would get a rank below 10000 at All India level in JEE Mains which ensures a berth in a reputed NIT. As far as IIT admission is concerned, a rigorous coaching is essential.