When you get there, go to some sort of shop/stall and ask the guy where to buy a sim card. He will then take you to where you can get one (they all seem to work together over there. If you need something, just ask the nearest person).
They then need to take a photo of you for some reason. Then you'll get the sim.
I was with Vodafone India and it was pretty cheap to make calls within and outwith India. almost 5 years ago