Who set the rules for validating a block in blockchain? Who set the rules for validating a block in blockchain? 

The rules can be set the creator of the blockchain network.
There might be some rules on the application that uses the blockchain network.

Is it true?

How does the rules setting works, how does validation work.
The rules are set by the nodes on the network. The rules are coded into node software which users run. These rules are not necessarily set by the creator of the network as they can be changed if all of the users agree to change the rules by running different node software.