Donatello did not know where he could earn money and went to a secret organization that provides erudite and highly classified mutants as hired workers for any dirty work, as a bodyguard and secret agents. They were paid decent salaries by the rich people they contacted. Donatello was trained and became one of the best, which is why your father chose him. Donatello wanted to take his brothers with him, but each of them refused for personal reasons.