Welcome to the Plone community, @newbazz. We are currently working on brainstorming ideas for the coming GSoC. We have a form you can fill out with your own idea, if you like. In general, you'll have a better sense of what kinds of work might need to be done (or what might help you) if you try to become a user of Plone. Set up a website with it. As you do, sketch out a list of things that are painful or difficult, things that confuse you or surprise you. Those things are generally good candidates for GSoC project ideas. Helping to make using Plone easier for everyone by fixing problems you find is a great way to get started.
If you want to dig in to learning the Plone code base, the page that @tkimnguyen linked above has pointers to our issue tracker. Picking up a "starter" issue and working on it can be a great way to get started. You'll need to ask questions, and that will help introduce you to the larger community. We notice who is working on Plone and engaging with the community, and that definitely plays a part in our choice of GSoC students.
Good luck, and let us know as you have more questions.