Prices on year by year basis I want to know the cost of milk on a year by year basis, say for the last 10 or 20 years. similarly I may want to know price of other items also (petrol, rice, biryani, etc). I just need the average values. What is the best source of knowing these things. I'm particular about India. Please give me any website name or so that provides such statistical data.
You might want to try the Indian Government's open data website https://data.gov.in/

I am not sure if all your requirement will be met, but this is a good place to begin your research.

To find the data you require, you might want to use the "search box" on the header.When I searched for "retail milk price" (keywords), I could find several datasets for different timeframes.