Who decides that the new block is ready to hash? I know that new blocks are relayed one block per 10 minutes but that time is not exact but average so: Who decides that the new block is ready to hash?
A new block is ready to hash as soon as the last block has been solved. Miners immediately start to try solving the next block. As soon as one of them succeeds, that block is published and all the miners move to work on the following one.

The difficulty is adjusted so that a solution is found by somebody, out of all the active miners, on average about once every 10 minutes.