You need a cloud based means to communicate and also a cloud based means to start working on the project. There are various tools for doing that and you might go through a few before you find a good fit.
For collaborating remotely, I use a lot of email, shared google docs, private blogspot blogs where I can invite other authors and even Twitter. Of course, Slack, Trello, Github and other tools are popular in the development community. If possible, use something familiar. Trying to learn a new tool can be a project in its own right. If it isn't critical to the success of the project, don't do it. It just becomes a means to procrastinate.
If you have nothing, start with a design doc. If there are no roles assigned yet, take the initiative. Set up a file, start a Google group or a Slack channel, list goals and start hashing out who needs to do what.
You need a central repository. It does not have to be fancy. As your project grows, layers of organization can be added as the need for such becomes clear.
Just start. No more excuses. Expect the first attempt to be ugly, bad and stupid. Do it anyway.
Best.
