If you want to earn that through online, the answer above is correct of being a freelancer. You don't need to invest any money on it as long as you already have the equipment ready like your PC/laptop and good internet connection.

But if you haven't checked your local place, you need find a job and save money to do other things that where you can invest your savings and generate a profit or dividends. Like if you have saved money and bought a real estate for rental, that's passive income monthly but as you have said, no investment needed.

You get only a few opportunities with that condition you have set.