Since you are in India, I assume you are asking about the laws there on use of a metal detector. I would suggest you look to see if there is a metal detecting club in Delhi; local clubs are a great way to get such information. I would think you can use one on private property, if the owner gives you permission. If you find a relic of obvious historic importance, I'd suggest you take it to the university or a museum to get an opinion on what you are responsible for doing with it.
Metal detecting is a great hobby, but it's always right to start with ensuring you are within the law when doing it!
Good luck to you!