Try to find a Mod out there - one with a team who is actually building something - and pitch them a few missions, maybe a story vision.
It's a little different writing well for a game, because you need to have you're fleshed out story-arc, which meshes with the gameplay, which can be brought in often enough that it moves the story forward, all without annoying the user. You're not writing a Novel, remember... :)
You'll probably get turned down at first at a lot of places (lots of people want to help with mods, but can't code/model, so they try to be writers...), but if you're actually any good then you'll find a crew.
Good Luck!