To start something good , no age and day is late ! Just start your preparations wholeheartedly and sincerely from day 1
Now the study strategy !! ( I followed the same and generally suggest others the same, because for me self study is the best 😂😝)
My dear for Biology Ncert should be your Bible.
For chemistry particularly organic and inorganic ncert should be your Bible
For physical chemistry and physics practice standard modules and ncert all questions whether of chapter end, examples,or in text questions as you have to practice a lot of sums and read theory portion
Try to solve last 10 years MCQ questions.
Before 1 month of exam try giving proper 3 hour mock test like exam so that you can be confident and well versed in your portions
Revise your portions daily by setting targets and give equal weightage to all chapters of class 11 and 12