I wanted to ask how can i start preparing for RMO ,INMO and IMO i have bought some books recommended .How should i start about with them..

@ ajay 
frst thing that u have to do is practice the previous year papers of all these exams . take at least 6 years previous year papers , and start solving them . 
and after that take out mtg books and start practice the model paper , by that only u can improve your score . 
and do revise things regularly , frst attempt a model paper , and then analyse the mistake that u have made in that paper . 
ALL THE BEST ..