Dear Aakash,
Important Topics :
All the topics are imporatnt .. i m giving the some topics on which u need give more stress..
In 12th topics , i wud suggest u dat modern physics is d most easiest topic in iit jee and u can score very high as its weightage is also good so if u can score in this topic den u r always better placed ,,,,,after this i wud suggest dat waves and simple harmonic motion r some easy topics from class 11, again for 12th ,magnetism & current r scoring topics..........
For math, calculus is the most easiest nd scoring topic in jee as it posses a high weightage nd solving questions from dese topic is very less time consuming,,,,Also, vectors nd 3 D are very much scoring topics.........Also for d last 2 yrs p&c is relatively easier....and other important scoring topic is conic section as it is easy n can be done easily.
All the Best !!!