Think like Seam - start with the entry point to the application. What happens when the listeners (web/jsf) fire? Bootstrapping etc. What happens when the request comes in and filters are hit? Contexts set up etc. What happens in the interceptors? How does the resolving work?
Then you can pick some small issue from JIRA, make a patch and send it to Pete (or someone he assigns)
Personally I started contributing to Web Beans since trying to learn Seam internals was like trying to drink from a fire hose ;-)
The fastest way to is just jump in. If there's an area of Seam you think could use some improvements, study the code while implementing that feature and then if you are happy with the result submit it to the project. If that's a bit too ambitious then perhaps look through open JIRA cases for something that is smaller. You can learn the code the same way we developed it - one class at a time. :)