I have not prepared anything for 12th board I was studying 11th portion .if I start from now will I be able to fetch at least 85 percent

go with concepts more useful than that byhearting rote learning books

Yes sure solve arihant chapterwise books

Yes u can .
It's really very easy to get 90% in boards .
U just need 3-4 months for that even 2 months are enough .
Just need to do ncert nothing more than that .
First make a list of chapters and then mark which one u know and the others u haven't even read .
Now first start with the chapters u haven't read yet do them clear ur concepts .
So that when u revise during exams nothing should be felt untouched .
Just do all the chapters one time and if u have time revise it .
For physics numericals asked are really easy and they are of 18 marks hardly . Rest of the paper constitute derivations only . so it's really easy to score in physics .
For biology just try to answer with diagrams and flowcharts .Questions asked are never tough u just need to learn how to present the answer .
For chemistry nothing is specific just study and u will be able to score really great .
All the best ....