My advice, don't. I wanted to sign when younger but I needed an adult so I got a parent to read through it. If your work goes big, you won't have rights to any profit it makes. They're the ones with the say to whatever it gets adapted to and you'll only get the rights to how it's doing on their site.
You have to hit word counts to get bonuses and if your story falls off, so does your pay. There's a whole lot of other stuff and it's all so disheartening that I wonder why some people get into it but I guess some are lucky.
If you want to keep the all rights and say regarding your work especially, don't. Hope this helps