Just start writing it.
No, really. I had a vague notion of where I was going when I started writing Swift as Steel, and I was seriously about 75k in by the time the last pieces of the plot clicked into place. This now means that there are a pretty good chunk of scenes that are going to have to be rewritten/condensed/torn out completely when I go into revision phase for NaNo this year, but such is life.
For me, the process of writing generates the story. I can plan out a general arc; these are the characters, X Y & Z bad things happen, but the actual details, the side conflicts, the villain motivations - those all came together during the process. (And in the shower. For some reason, I get some of my best solutions to things when I'm in the shower. I've no idea why.)
Start with an idea. A seed. Start playing with the characters and their lives and watch what happens. They'll start telling you who they are and what they care about. Then all you have to do is start throwing monkey wrenches into their lives and the plot will appear.