I'm don't claim to be a good author (so take this with a grain of salt), but maybe the first step could be deciding whether it should be character-oriented or plot-oriented?  If the focus is more on the characters, you should look for a simpler overall plot to take a backseat to character oriented subplots.  Perhaps you could also decide what world it takes place in? Earth, fictional, history, now, future, etc.  Then, maybe focus on the orginization itself.  Are they fighters, cunning spies, or meeting in secret for some sort of activity?

I don't know if this is helpful or not.  I guess my point is that you can structure and build the other parts first and sometimes the thing that you're stuck on can follow.   Good luck!