I'm am going to write my 11th board's in march. I badly want a seat a iit bombay for cse. I do not have any reservation. I realized about the hardwork needed for it .can you tell me how I can get a rank below 100 for this . Is it possible?

Obviously it is possible but not so easy. The competiton is very much so you have to work hard. To get under 100 you really need to work with punctuality.

Your case is good that you are only about to give board exams.

I would suggest you that if your concepts of class 9th & 10th are not clear than forst of all make them all correct in the month of april-may.

After that read NCERT books of class XIth very deeply. Than refer some competitive books like DC Pandey for Physics RD Sharma for math and many other good ones are there. Refer any of those. 

You can also join coaching centres. But ultimately its you who have to work hard. A coaching centre cant make you select in IIT-JEE.

Best of luck for your selection.