(1) It's really all about trust. The client wants someone who can reliably solve their problem, and tooting your own horn has a very limited effectiveness in building that credibility. Trust heavily impacts what projects people will give you, and how much you can charge. It's common that a company will somehow find 2-3x budget to hire a partner who they know will get the job done.
(2) Referrals and good deeds are the fastest way to build trust. When you come well-recommended by a prospect's trusted friend or partner, then a good amount of that trust gets instantly transferred to you. This transitivity of trust is key to building a good referral network that will consistently send work your way. If you don't have this, then you have somewhat of a cold start problem. In this case, providing value to people on a regular basis could really help with establishing your credibility. I don't mean doing projects for free, but more like offering people free 30-minute consultations about how to build their things, or sending them resources (articles or books) on a consistent basis that would really benefit them. This demonstrates that you can already deliver value, and makes it more convincing that you would do much more of that if you actually got paid for it.
Here are some more resources that could help someone get a start:
 Getting Started in Consulting by Alan Weiss is a little antiquated, but talks about what's important in getting your firm going and how to think about your work's impact on your client. (https://www.amazon.com/Getting-Started-Consulting-Alan-Weiss...)
 Book Yourself Solid by Micheal Port talks about the best ways to build these client relationships that will result in trust. (https://www.amazon.com/Book-Yourself-Solid-Reliable-Marketin...)
 Double Your Freelancing by Brennan Dunn actually has very good information about the tactics of pricing and the business side of project management. It's pricey. The accompanying podcast has good information for free.
There are thousands of different info products online about this stuff, but this bundle should give you the most core knowledge for your money.