How do I change this direct speech sentence into reported speech?

Ram says,”It will rain tomorrow.”

Reported speech always takes a step back in time from direct speech. Present and future become past, past becomes past perfect. So the correct basic reported speech version of your sentence is:
 

• Ram said that it would rain tomorrow.

 
But, this would only be correct if the speech was reported on the same day on which it was actually said. For example, Ram says on Wednesday “It will rain tomorrow”. Then, you tell Sarah on the same day, what Ram said. It becomes: Ram said that it would rain tomorrow. If you told Sarah on a different day, say Thursday, then it would be different. Then you would say:
 

• Ram said that it would rain the next day.  or 

• Ram said that it would rain the following day.

 
As a quick reminder, in reported speech:
Will becomes would.
Says becomes said.
Tomorrow becomes the following day/the next day.