There is a plethora of online resources regarding working with Chef; here are some key sources to get you started.
These are resources that are made available by Opscode and focus specifically on Chef itself.
Opscode maintains a wiki that serves as an official resource for documentation; as new features are released you can find current information here: http://wiki.opscode.com/display/chef/Home
You can download the source code of Chef from GitHub at https://github.com/opscode/chef.
The community portal available at http://community.opscode.com/ is a good starting point for finding community-provided resources, events, and updates.