First and very important rule, if you want to participate in GSOC, choose some 3-4 organizations from the list of previous participants (Its very probable that they are going to be selected for next year too) and start playing with the tool/application i.e. install, configure, run on local, run for your own test cases etc. The idea is to evaluate your assumed interest for that software and if it turns out to be interesting you would end up being comfortable with it.
After that you need to prepare a proposal and submit it