hello sujata,

first you can get a good hold on the NCERT of 12th and then for further preparation go for the previous year and sample papers.

that too, depends on the exam you are apllying for.

refer to this BHU official link for some question papers

http://bhuonline.in/old-qus-paper-entr.html

and get books from flipkart and such practise papers for better results.

good luck