If you haven’t already, check out our current list of ideas here: https://github.com/AnalyticalGraphicsInc/cesium/wiki/Google-Summer-of-Code-Ideas-2015
Start by following our Contributor’s guide to get up and running with Cesium development : https://github.com/AnalyticalGraphicsInc/cesium/wiki/Contributor%27s-Guide
You can pick an issue from our list to work on: https://github.com/AnalyticalGraphicsInc/cesium/issues to get started. There is a “beginner” label for some issues, but feel free to work on anyone you think you can fix. And feel free to ask questions on this forum about whatever you are working on. Open a Pull Request when you are ready, be sure any required documentation or tests are written as well. While contributing to Cesium before hand doesn’t guarantee you a GSoC spot, it does go a long way to helping us see what your skill level is and increases your chances of being accepted.
If you have any specific questions about any of the projects, or want to propose your own idea before applying, just use the forum.