The best way to prepare for applying for mentoring programs like GSoC in the future is to become a contributor of the project that interests you ahead of time. There are thousands of open source organizations out there, and hundreds of them participated in GSoC in the past. (For example, see the list of organizations that participated in 2014.) You may be able to find a mentor within a particular organization.
Once you decide what project you are interested in contributing to and explore the information available about that project on its website and wiki, you can introduce yourself to the project's mentors and ask them any questions you have about contributing to the project. The mentor can help you build the project's code, identify an easy bug to start with, and help you with your patch for that bug. The mentor can guide you through your subsequent contributions and point to the resources for solving a particular task.
IRC is one of the main communication channels for open source projects. You should try contacting a mentor in the project's IRC channel, and use e-mail if you don't see them on IRC. Typically, there are other people in the project's IRC channel, who can help you too. You can address the mentor directly in the IRC channel by using their nick in your question. E.g. if the mentor's IRC nick is kelly, you can say "kelly: hi! I just built project-foo and looking for a bug to fix - I found bug 1234 and bug 4321 in the project's bug tracking system that both look like something I can try to work on, but I wanted to see if you have any recommendation, since you are listed as a mentor for the project".
Please don't hesitate to explore an organization even if it's not obvious who to reach out to. Chances are you will find some clues about how to get involved on that organization's homepage. One approach is to look at the commit log of the project within that organization that interests you to see who are its most frequent contributors and follow their posts on the project's IRC channel and mailing list to learn more about the project. You can also ask any questions you have on the project's IRC channel and mailing list.
You can find more mentors, easy bugs, and things to learn on the OpenHatch community site. In particular, OpenHatch training missions are useful to complete for anyone new to contributing to open source.
If the organizations participating in GSoC this year haven't been announced yet, don't let that stop you. Even if you start out contributing to an organization that happens to not be part of GSoC this year, you will still be much better prepared to apply to a different organization that will participate in GSoC.
Please help spread the word about this resource by sending a provided e-mail to student groups, posting a status update on social networks, or distributing the flyer and the leaflet with the information seen on this page.