I am in 12th right now. How do i prepare for IIT exams next year. Which books should i buy or Which topics should I concentrate most on.

dear lara , these are some gud books

for maths - A das gupta IIT METHAMETICS

for chemestry - P bahadur , arihant organic chemestry

for physics - arihant DC pandey , HC verma vol1 & 2

these are sufficient to crack iit with a gud rank ...

all the best