According to me if u want to do or want to crack iit jee then u have to give ur 2 years life fully devoted to it means 10+2 u should focus only on study nothing else then there will be chances to do something good in this examination.
If u join some coaching or course then u should be devoted 1000% towards it if ur focus is gone for some day ur whole preparation wills suffer so be patient and do hard work if u want to do coaching after 10+2 then according to me u should not join any coaching make a tym table sit at home and study for entire year honestly results will be better because in coaching they will give u all that thing which u have already studied in 11 and 12 so revise it with different set of books join some test series to see how well u can do so u have to work hard for this thing and hard work required do so.