Hey Buddy, first of all, remove all the tension and just relax. You will not be able to study if you take too much tension. Be calm and have a positive attitude.
Start your preparations from now itself. 11th ISC is not that hard if you study on a regular basis.
Make a schedule/timetable for yourself and study rigorously according to that. Make sure you go with the flow of studies. Study according to your syllabus and exam pattern.
Practice previous years question papers. It will help you boost up your confidence. You will get to know the question pattern and also where you are lacking and what are your weaknesses.
You must at least study for 2 hours on a daily basis. This will help you a lot and you will not forget anything easily. Also, always write and study.
Never try and sit with one subject for the entire day. Mix-match that is, pick two subjects for a day and work on every subject with divided time. This reduces the boredom as well as enables you to finish the portion simultaneously with all subjects, in a planned way.