What is the source of initial transactions of bitcoin?
What is the total amount of all those transactions


You can see Bitcoin's genesis block here:

https://en.bitcoin.it/wiki/Genesis_block

You can easily examine early blocks by using the block height in any blockchain explorer. For example here 


  how they got BTCs for the initial transactions?


Mined blocks can be empty (contain no transactions other than the "coinbase" mining reward). I guess this can be the case when the mempool is empty or when all the pending transactions therein have unattractive fees.