- Edited
Empe_ror99 I agree with this one.
@popular_ruki Writing fanfictions - or even reading them - is a great start, if you're not too much into reading lots of other novels. I've read a lot of fanfiction works that can easily rival pure fiction ones both in writing styles and plot. Writing them can also help you build a starting network of readers for your work.
For me, confidence is something that's earned whenever you accomplish something. It's not something that can easily be built from thin air. I suggest focusing on goals that make you happy. What really helped me build enough confidence to share my work was when I was able to write a plot that I hadn't seen in any novel or fanfiction I read before. That was a huge achievement for me, and I was happy enough to want to share that work with others who might have been looking for the same things I was looking for in a plot. Readership numbers and feedback (I mean genuine reviews and comments) built my confidence later.
So how does one know that his book is good? To me, it's when the author themselves think that it's a good book. If it's something that the author will enjoy reading again many years into the future. Accepting reviews and feedback is good, but don't purely rely on them if you're trying to build your confidence. Taste in novels is subjective. No matter how beautifully written a story is, there will always be someone out there who will rate it as crap for the heck of it.
So write something that makes you happy; learn from the feedback and ratings you receive if you decide to share your work with others; and write something from what you learned that can make you feel even more accomplished and happier than the first novel you wrote.