Don't start. Asking "where do I start?" is a bad sign when your only goal is to make money.

There's nothing wrong with making money, of course. But any project must solve a problem or create something of value. Right now you haven't identified anything to do worth doing.

A startup isn't going to succeed unless the person founding it knows what he's starting and why. Wanting money may be your justification, but why would any customer care about that?

Continue working in areas where your work is valued. If possible, convert those jobs as an employee or freelancer into your own ongoing client base. Later you can build a brand for yourself around that ... as your actual business grows.