Shinichi, creating an RPG is an acquired talent, that more so requires creativity than writing. Before you work on an RPG, which requires writing a story basically, a plot, you should more focus on creating the character, the sign up.
Those two things are completely different. GMing an RPG is a rather difficult task as well, it requires much time, you do A LOT of work for GMing an RPG.
I can help you, give you suggestions, but I can't write you one, then it becomes not yours but mine.
If you really want to know how to create an RPG, this is what you need to do:
1. Come up with a setting, plot, and story
2. Explain the setting, plot, and story, in about 500 words minimum for plot.
3. Take the story you wrote, and explain it to your audience and readers, to make it understandable.
4. Create the Sign up platform. Psychic's "Making a Sign Up" thread can help you there.
Thus you have an RPG.
I come here to help you not with the RPG however, but the making of sign ups, as that is what you should perfect before you start GMing. Everyone has started off writing somewhere down the line, you can't expect things to be handed to you however.
It is a Dog Eat Dog World out there in the Writer's palace; many people fall and crumble; others succeed to great fortune making living legacies. How you decide to write dictates where you will be placed. Your RPG is your own idea, so asking people to help create it for you is same as asking one to do your Home Work.
I will help you if you have questions, but you need to ask me said questions