Hi Komal
Though your question is not clear but I'm assuming that you want to know that how to get good marks in 12th to get admission in colleges.
I would suggest first of all you should be prepared of the course you want to opt after 12th.
Then go through the colleges and check the admission criterias and exams required and then you should plan your 12th grade as required and focus more on the subjects required in future.
Good Luck !!!