Contributing to Crosswalk
The first step to contributing to Crosswalk is to get a build environment configured and operational. The Building Crosswalk page provides that information. In addition to the binaries Crosswalk provides for the supported operating systems, the build infrastructure can generate projects for additional platforms.
Once you have a build environment, familiarize yourself with the debugging process. There are steps describing that process in the Crosswalk Developer's wiki page.
Now you just need to pick something to focus working on. You can look through the Issues, or hop on IRC and ask what people could use help on.
Be sure to follow the contribution guidelines as you develop your changes. By submitting your contributions to the Crosswalk Project, you agree that your contributions are licensed to Intel under the terms of the BSD 3 Clause license. If you do not agree to these licensing terms, please do not submit your contributions to the Crosswalk Project.
Full details of everything related to test scope, test execution plan, and running tests can be found on the Crosswalk testing home.
You are welcome to contribute test cases for Crosswalk Project, the detailed process can be found in contribute tests page.
Jump on IRC and chat with other developers; they hang out on Freenode in the #crosswalk channel. Details in the Community page.