Have you joined any coaching institute(classroom) ? If yes, then just keep up with whatever is being taken up there. 
Otherwise, you should certainly take AT LEAST one good test series (FIITJEE's AITS and BT's BMAT are good). Then, plan yourself so as to finish at least 80% of the syllabus before the test series starts. 
You have a lot of time, don't worry too much. Be regular. Regular and devoted study of 3-4 hours daily should be enough to get you through. Spend 1-2 hours to make a plan, make targets according to meet that plan, and never ever think of making a new plan. For example, you can make a target that you have to finish 70% of Class 11 syllabus before your school half-yearly exam etc. Make your own target, according to your needs and priorities. Don't neglect school, and do take breaks in between....have fun and dont get burned out. 
And I would strongly recommend joining some good coaching class. If not, buy coaching books second-hand or even better, join correspondance course from a good coaching center. 
Good luck!