sir i wanna crack iit jee and want to take c.s. branch in iit mumbai...what minimum rank i should get? and how should my preparations for that?

Your rank should be in top 300 (atleast) then only u can get this branch. In 2012 about 86 students of top 100 went in IIT Bombay and most of them took CS Branch. For CS Branch in IIT Bombay u have to be the best

your maximum rank should be upto 700. for that you must prepare like the one who is air 1.
good luck with that.

under 85