A few useful hints for those looking to participate...!
You can browse the threads from last years by using the tags:
Most notably some of the students have written up very nice experience reports as they finished up their projects, here are a few:
- Bit Array and Bit Set API Review (The end of a GSoC Project)
- GSoC – Shared storage for property wrappers and @expanded parameters
- Google Summer of Code "Improving Swift’s Debugger experience on Linux" Final Report
- Current state of Swift Tracing
They contain useful insights and retrospectives about the program from past students. They can be useful to wrap your head around how to approach all of this. The most common hint I believe is to not be too shy to ask for help or clarification -- work with your mentors to deliver the best project possible 
We're looking forward to your threads and proposals!