Find a job (part time or full time) that pays in Bitcoin
Start a business and have Bitcoin as the major payment system.
Do freelancing works that pays in Bitcoin
Offer whatever skills you have and accept Bitcoin as payment
You can mine for Bitcoin

Last but not the least,
You can also buy Bitcoin in exchange of fiat.

Consider Bitcoin as the fiat money you have. How do you Increase (earn) fiat?
It's the same for Bitcoin.