TFS Example - best process of starting a new revision of an existing project

I am looking for an example that shows how to start a new revision for an existing project.

The Project already exist in the TFS system (features, backlogs, code, tests). Now I need to add some more features to the existing project which was already released. How start the process (after all there is no need to create a new project). How to differentiate the new code from the old one and how to run the updated set of tests on the updated version (and to be able to trace back what was run for the previous version)

Thanks 

Beni