How can i study 12 organic …

You should be thorough with class 11 organic chemistry chapters like naming of organic compounds (as you will find it difficult to understand the name of a compound when it comes to 12th) and reaction mechanism of class 11(it is the base chapter which contains topics like resonance/mesomeric effect,hyperconjugation,electromeric effect,inductive effect,substitution reactions,elimination reactions,reactions of benzene plus some rules like Markov Nikov rule and Anti markov nikov rule,which are very very important for your class 12th organic).So you should cover up these topics to get the base for class 12 organoc chemistry as all the chapters related to organic in 12th purely require the idea of the above mentioned topics.So clear your concepts and wishing you all the best......

But which topics i have to cover to start with 12th organic?

It is very tough to study class 12 organic chemistry if you didn't get the base ideas of class 11 organic chemistry.So I would recommend you to study class 11 organic chemistry first and then move on to class 12 as it is very difficult to understand if you don't know the actual concepts behind it.