To calculate difficulty of a block we need the parent block timestamp and difficulty? As there doesn't exist a parent block so what values should we take for those?
The difficulty for the genesis block, along with all the other parameters, is hard coded into the Ethereum clients. It is not calculated. More information about genesis block can be found here.
The Genesis block is pretty much a database file: it contains all the transactions from the Ether sale, and when a user inputs it into the client, it represents their decision to join the network under its terms: it is the first step to consensus.