Some of the rich folks where he worked had Egyptian souveniers, and he became interested, so when one group needed a person to copy the wall paintings inside tombs and couldn't afford a professional, they hired this lower class 17 year old instead.
The History blog has a nice summary of his work and career.
One example of his paintings:
Once there, he learned a lot from other Egyptologists and started to do excavations.
He later branched into administration as an inspector, got fired for telling off pushy tourists, and then sold paintings to stay alive. Later he went back to excavation and met Carnarvon, and the rest is history.
A painting of Hatsheptup's temple