DynamoBIM is the main website for Dynamo. Here you can download the latest version, run through some tutorials and join in the forums for advice. The DynamoBIM forums are the best place for quick information, visited by a heap of expert users and the developers themselves.
The Revit Dynamo forum is basically what it sounds like, a forum for Dynamo. This forum is okay but a fair bit less active than the DynamoBIM forum. Still, some great advice and knowledge here.
I can’t praise Code Academy high enough for a place to learn about programming. This applies to Dynamo if you want to get a rundown on the essentials for Python programming language. Python can be used for all sorts of good stuff in Dynamo so I highly recommend using this resource to get the best intro to it.
Revit API Search has been set up by the Dynamo Team and contains a complete, searchable index of all Revit API namespaces and methods. It’s a great site to quickly jump on to and browse the API if needed, definitely helped me out early on. Keep in mind though that it is the API for Revit 2014 so there may be a few changes depending on what version of Revit you are using.