Hii!
Since you are still in class 11th, the only way to be prepared for future and board exams, is to be consistent in your Studies. You need to revise what's taught in class every day. Solve question banks and previous year question papers. Revise each day, take help of your teachers to solve doubts as soon as possible. Study everyday, fix timetable for it. Focus on the subject you find tough, from the very beginning. This will help you score well in it. Also, make small notes, as you study.
The only way to to prepare is to keep on practising.
Hope this helps.
Good luck!