The question should be "What does a lay person have to do to become a priest?" or "What did a priest have to do to become a priest?"
Depends on the religion.
- Some priesthoods are hereditary.
- Some are due to reincarnation
- If you start your own religion you can self declare
- Others are faked from a diploma mill
- In Christian churches traditionally there is instruction in a seminary, psychological tests to weed out the inapt, the criminal and the insane, a serious amount of internal questioning and some formal vowing and blessing