Building a help system for a saas application.

I have the job of building a help system for a not-too-easy-to-use saas application.

I am restricted insofar as I cannot make changes to the code or layout of the system.

Right now there is a very high learning curve to use the system.

Where can i learn about the best practices for explaining how to use an online service?

Does an interactive forum outperform a deep FAQ? Any services, like Zendesk, that you guys recommend?

Where should i start?