Below are a couple of links that describe the process for getting your project started using Collaboration for Revit (C4R).
It all begins with creating a project using your A360 account/subscription. Naturally you've got to create an account first so this assumes you've done that. The linked page also explains how to upload your current project to the A360 project if necessary.
If you're responsible for putting your active file on the A360 Project, READ ME, it has a video too.
There is a ton of information lurking at Autodesk, just use your Google-fu.