You need to figure out which one you want to work on, then write a proposal and submit it to the GSoC process (which I don't have any knowledge of).
I would suggest checking out the pgAdmin code from GIT, and making sure you can run the Python/JS parts on your system (you might want to work in PyCharm), then start working to understand the areas of the code you'd be working on. When you have specific questions on how to build/run or how the code works, feel free to post them here.
