The introduction is a good place to get started with Goa. It provides an overview and the rationale behind the framework.
The Getting Started Guide walks through implementing a Goa service. It provides an overview of the design DSL and walks through the steps involved in designing and implementing a service using Goa.
The Upgrading from v1 to v2 or v3 document describes how to upgrade a Goa service from Goa v1 to Goa v2 or v3.
Consult the FAQ for a list of commonly asked questions.