One mistake I think a lot of new writers make is trying to take on too much too soon. They outline sweeping stories with multiple sub-plots, huge casts of characters, deep, meaningful themes. They choreograph complex fight scenes and build complicated worlds. And all these things are fabulous.
If you're up to it.
The fact of the matter is, most beginning writers aren't up to it.
The book of your heart may have one or more of the above components, but if you don't have a few finished, simpler novels written and under your belt, you may not be ready to write that heartfelt book. It might not be a bad idea to put that one on the back burner. Try something with two or three characters, one storyline, set in present day, requiring little research. Baby steps. Makes sense.
I wrote three novels before the fourth one landed my first agent. And yes, that fourth novel had multiple points of view, a complex dual plot alternating between the main character's past and present, and some complex world building. It was the book of my heart, the character I wanted to focus on. Could I have written it better if I'd had a few more practices beforehand? Absolutely.
I knew going into it that even with three previously completed manuscripts, I might not have the chops to pull off what I wanted to accomplish. So, I fell back on something that gave me a ready-made foundation: fanfiction.
For those unfamiliar with the concept, fanfiction is playing in someone else's world, or with someone else's characters. It takes one or more items out of the writing equation. You don't have to build the world if it's already built. You just have to add a new story. You don't have to establish as much characterization if everyone already knows the characters. And often you don't find yourself over-characterizing because YOU already know the characters.
Writers can use this medium as a practice ground. And there's some fantastic fanfiction out there. I've read great Xena stories on fan sites. The science fiction/fantasy/superhero shows and comics are frequent subjects of fanfiction. Many established novel series have fanfiction short stories written about them. And posting the work online can earn a new writer some valuable feedback.
Now, some might view fanfiction as plagiarism, but I would hope most would find it flattering. If it were me, if I had something published that people wanted to write fanfiction about, I'd like to believe I'd be flattered so long as they weren't making a profit from the work or trying to pass it off as something I'd created.
So anyway, I used fanfiction to help me work through some characterization issues in the book of my heart (and one more since). I won't admit here which worlds I played in, and I'd challenge anyone who doesn't know me personally to figure that out since I made dramatic changes, but I had previously established people in my head. I definitely used the medium, and I highly recommend it for beginning writers. Because the cool thing is, once I'd done it a few times, building my own characters came that much more easily to me.
If nothing else, it makes a great practice exercise, whether you share the work with anyone or not.