No worries dear ! 😌
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