The Central Groundwater Board have come out with watershed and basin-wise maps for the entire country. Here's an introduction and the link to the resource
Using a rain barrel is the easiest way to begin rainwater harvesting. Rain barrel serves as a storage unit for rooftop rainwater. It can collect a substantial amount of rainwater.
This is a technical manual for rainwater harvesting from the Rainwater Club, giving detailed information including required parts and approximate costs.