Well, I do actually have some experience with book to film adaptations and it's not an easy thing to manage. You don't simply get an adaptation because you want one, you get one after proving the story is worth the investment. So, what's the process like?
First, you have to have a script, then you have to find some way to get a foot in the door. Most writers would depend on their agent to get some eyes on the script. If you don't have an agent or a company backing you, then that makes it more challenging.
The other way would be to submit a pitch during the rare times studios are open for submissions. They usually have some things they are looking for so your story might not be suitable for every opportunity, for example, if you write a fantasy story and they're looking for horror.
Even if you have a story being shopped around by an agent, getting someone to greenlight an adaptation can take time, even years, if it doesn't fall through. That's even worse of late with studios not wanting to take as many risks. They are more likely to invest in franchises with established fanbases and bestsellers. They want a ready audience to feel more secure before investing.
Writers who have a movie made from their work are usually first paid for the option rights, which varies. This is what a writer is paid to agree to have their work shopped around to studios, regardless of whether an adaptation is secured. The amount varies, but it's usually no more than a couple of thousand. If an adaptation is greenlit, you get paid for film rights. How much you get depends on how much the movie budget is and how big your book is, so a bestseller will get a lot more. It really depends on how good you and/or your agent is with negotiations.
Basically, lots of people have good, even great, ideas, but we don't get to see them adapted because most people don't have the connections and resources to facilitate this. Even those who have a chance don't always get a movie deal, and even if they do, it doesn't always guarantee a secure job and continued success.