I have secured 6600 rank in jee advanced what should i do now i m confused among prepare for next year or go for BITS PILANI or get any branch in iit

first of all, congratulations that you secured a rank in jee advanced. 

I would personally suggest you to go with taking  a core branch in BITS Pilani if your BITS score is very good. with a rank of above 6500, it is difficult for you to get a core branch in any of the iit''s and ism dhanbad. but still, it would be good if you go through the various branches in iit''s and ism and attend the jee councelling and test your luck. 

all the best