To earn bitcoin, you need investment in any case. Because you buy bitcoins to make money on their growth, or you spend money on buying and maintaining equipment for mining.
There are ways that you can earn even you don't have investment to put in bitcoin. But you need to invest somehow in internet, electricity and other utilities to market and sell out your skills to the companies. You don't have to invest directly in mining as bitcoin mining, for me isn't profitable anymore. Just be a freelancer.