Quote
32 merkle_root char[32] The reference to a Merkle tree collection which is a hash of all transactions related to this block
~~~~~~~~~~~~~~~~~~~~~~~What's this? where does it come from?
That ones a bit confusing and has to do with verifying all the transactions in the block more efficiently. google "merkle tree".
When mining, how to fill this field?
Okay, how do we fill the tree leaves?
Transactions.
I don't understand the concept, we will have transactions after the block is found, right?
block is used for transactions.
What transaction do we have before the block found (while mining)?
Blocks contain transactions. The fact that a block contains them "verifies" them. A transaction is generated by someone saying "I want to send coins." Have a look at
http://blockexplorer.com/, gives some pretty useful block data.