I make a fair chunk of my income online, and most of is it what could be classed residual. There are things I can actively do to promote it and increase the income, but if I did nothing at all, there would still be some income. Enough for someone to live a simple and frugal life, growing much of their own food and harvesting their own wood for cooking and heating.
For most of us, starting to earn passive income is what Paul said:
Help people, online, for free. Anywhere. In any way. On your favorite topic(s). Be thorough and magnificent. In a few months, go back and harvest the things you did to help others and see what you can cobble together into passive income.
To start, ask yourself what you're the go-to person in your community for - in your family, your friends, your neighborhood, your church, your favorite online forums? What are the problems people come to you seeking help with? What are the recurring patterns in the replies and the support you give? That's your area of expertise.
Then it's a matter of packaging your expertise, into blog posts with affiliate links, an ebook, a video course. There's a time investment, but most of these can be done with no or minimal expense. Now you have a product to sell or to give away.
People hold back because they don't think what they have to offer is good enough, or they see the market is already crowded with similar books or courses. But you have something unique to offer. Maybe that's a way of doing things not many other people are trying. Maybe that's a particular set of challenges you've had to overcome. Maybe that's something from your own unique life experience that helps give you a different slant on things. Whatever it is, it's your unique selling point. If you don't know what that is, ask your friends.
As for not being good enough - maybe it is, maybe it isn't. We're not the best judges. If you aren't sure you can charge money for what you have to offer, give some away. Ask people what they think of your book or your course or your site. And when it's enough -- not perfect, but when you have a minimum viable product -- put it out there with a price on it and see what happens.
The main thing is to start! Just start. I write fiction, and launched my first book 6 years ago. I don't earn a lot, but I've built up a readership. What I earn would be enough to get by on if I had to. Okay, if the economy completely crashes, maybe I won't earn enough. That's why I'm squirreling away what I can now and intend to buy land as soon as I can. But I have friends from years before I started seriously writing for publication who also taked about doing the same thing. They're still talking about it. So start now. Don't be the person still talking about doing it in six years time. You earn your first dollar by doing something.