I don't know whether you're the kind of writer who prefers to start writing with absolutely no idea where the story is heading, but I find that if I start like that I usually end up not knowing what to write next and begin on yet another beginning. Even if you don't want to plan meticulously, having a vague idea of some major plot points can really help - it gives you something to be writing towards. When I have an idea in my head I usually focus first on the characters (which I think you've done, too) and then I really 'get to know' them, figure them out until I know almost every little detail. Of course I don't, and I get to know them as I write, but through getting to know my characters I find that the outline of a plot forms.
What matters most to you in this story? Why are you writing about these people? What are you trying to say? What will make their lives hell and heaven and at which do you want to end? Ask yourself a million questions about your idea and it will really help - you may get a chapter 1 from this idea (and it could be a marvellous chapter 1), but other than that there isn't much else to go on.
Good luck, will look forward to reading it.