You can use Scrivener, Writer's Cafè, or many others to create outlines. People use to love the first one, I liked the second. You can also use mind maps like Freemind, Mind Meister, SpiderScribe. I won't advise them, since I really don't like mind maps because they are - for me - too limited.
There are many ways to share your manuscript with coworkers. DropBox is an easy choice, but you can sync with Google Drive, for example. There are so many choices.
I prefer to work with Dokuwiki to create all the plot lines and sheets I need, since its wiki ability to connect everything works a lot better - at least for me - than more specialized software. Also, since I have my own domain, I have my wiki online and can share it with people I want since it already has an ACL system. The down point is that you need to have a minimum or technical skills - or at least will, since it's not difficult - to install and configure it.
For writing the manuscript itself, I use pure text formats - Latex or Markdown - and keep track of what I do with BitBucket. It has its learning curve but, once you learn how to use it, it totally worths the effort.
As a small bit of advise, sometimes you may have the beginning but not the end of your story. Some people thinks the best way is to wait the end to come from the muse before to start but I don't think it's the right way. For me, as you start to work the ideas will eventually come up. If you wait... Well, I guess most of the writer's blocks happens on those people that decides to wait.
In my own experience, if you start to work the plot lines and the ideas you already have, you will be able to beat inertia.