How sign the first transaction's signature? In bitcoin, each transaction's signature is signed with previous owner's private key. Then, how sign the first transaction's signature?
See Bitcoin Wiki.
Above image illustrates a byte-map of transaction with each type of TxIn and TxOut.

We should attention at "Coinbase/Generation" (Coinbase means the first transaction in a block).
Codebase's script is a arbitary data.
Yep, it is random.

Therefore the signature of first transaction of a block is not used.