If you want to write anything, but particularly a book, all you have to do is start.
Don’t worry about if what you say matches what you want to say just yet, that comes with rounds and rounds of editing, which may seem daunting, but don’t let it. Editing is the reason your favorite story speaks to you the way it does; I guarantee that the first draft is so much different than the final, and your book will be no different.
The most difficult and time-consuming part is getting the words on the page, especially if you’re a perfectionist who believes that if it isn’t perfect the first time, it’ll never be perfect. That way of thinking is a bigger hindrance than your writing skills or lack thereof, because an editor is going to do a lot of work for you in choosing the proper word choice and matching your description with what you’re thinking. And if you can’t afford an editor, you’d be surprised what edits you’ll make after you’ve read everything over a second time, but you need to have a draft to even get to that point.
You might surprise yourself with how well you express your ideas. The point is that you’ll never get to the point where you can read over your work or hand it off to someone else who can read it over if you never start. Don’t worry about grammar, spelling, pacing, flow or character development, don’t even worry about writing everything in order. Sometimes you have to start with the part that excites you the most and work from there. That might be the climax of the story or even the ending, and that’s okay. As long as you begin writing you’re already more than halfway there.