Not journal articles but some of the books I read as an undergrad like In the Name of Identity and How Wars End would be good summer reads. They won't give you a leg up on theory of philosophy per se but they are books that you can easily read and follow by yourself. The major works of political philosophy and current research would be better to read with a professor for context.
I also recommend the Model Thinking MOOC on Coursera.
If you really want to start reading on your own, many PhD programs have reading lists that candidates are expected to read by the time they complete the degree. I've gathered those and citations in articles I've used for research. I'm heading into grad school though.