Question: I'm from South India. My home roof is built with steel panels. During the summer, the roof will get too hot. I don't know much more about how cool in natural way.

Is there a way to cool the roof naturally?

Answer: 
Paint the roof white or silver to reflect as much sunlight as possible. This is a passive method once it is done.

Water removes heat better than air so if you have a source of water, or collected and stored rainwater, it could be sprayed on the roof during times of high heat.

This isn't "natural" but if you have an attic, you could install an exhaust fan with a thermostat switch that comes on when the attic gets too hot.

Good luck!
What you want to do is to prevent the heat from the hot roof from flowing into the living space. In the US traditionally this was accomplished by insulating the ceiling of the living space. The attic (aka loft) would get very hot but the insulation on the attic side of the ceiling would prevent heat flow through the ceiling. The attic was vented low and high to promote airflow to reduce the temperature of the air in the attic, but it was nevertheless hotter even than the ambient outdoor temperature.

The next step was to reduce the temperature in the attic by reducing heat flow from the roof into the attic. The main current technique to accomplish this is to place a so called radiant barrier to the underside of the roof. Another approach is to insulate the underside of the roof usually with spray foam, but also by attaching insulating batts to the underside of the roof.

In India and in parts of the US one has to be very careful that in attempting to 'improve' the utility of the structure for its human occupants one does not provide increased harborage for rats. Also one must not create spaces which trap moisture because this would cause rot. The science and art of insulation is very concerned with allowing trapped moisture to quickly escape so interior spaces do not stay moist. 

Does this structure have an attic that is accessible for addition of insulation?