To get some experience of working with wxWidgets code base we ask the potential applicants to make some simple modifications to wxWidgets. Please don't worry if you hadn't done it before, it's really not that difficult and we'll be there to help you with it. The important thing is to try to do it and see that you can, indeed, succeed as you will need to be able to work on wxWidgets sources during your GSoC project.
To get a general understanding of how submitting patches works, please read our patches HOWTO. Next, you need to select a task to work on. While you could take any open ticket in our Trac installation, we have a special list of simple bugs which are supposed to be easy to fix so we recommend that you look at one of those. Please feel free to choose one of the bugs that either corresponds to your experience (e.g. if you are a Windows programmer you could additionally filter by wxMSW platform) or have some relationship with the project you plan to propose. Finally, if you can't find anything at all for you to work on in this list, please ask us and we'll try to find something appropriate for you together.
Once you have chosen the task you are going to work on, please make your changes and prepare a patch as described in the HOWTO. You may work with either Subversion or Git, depending on what you are more comfortable with, but please do use a checkout of the latest sources as, again, you will need to be able to work with them during your project.
Finally, attach your patch to the Trac ticket, creating a new one if necessarily. As before, please don't hesitate to ask us on the mailing list if you have any questions about this procedure.
And good luck!