Wikipedia and the Stanford Encyclopedia of Philosophy are excellent sources for overviews of particular views, philosophers, and traditions. There is a series of "Very Short Introduction" books that are usually pretty good that cover an awful lot of topics in science and philosophy, and I'd imagine there'd be something there.
Resources can be of varying use depending on how much you're interested in learning. If you just want a broad overview, then these resources can be your end-all and be-all. If you're looking to learn more deeply, however, and are just looking for somewhere to begin, there's little harm in jumping directly to the primary classics, especially if you're willing to read patiently and slowly. Those would probably include texts like Rousseau's The Social Contract, Macchiaveli's The Prince and his Commentaries on Livy, Plato's Republic and Aristotle's Politics, The Communist Manifesto and Das Kapital, the Federalist Papers, anything by Foucault, the list goes on. Of those, if I were recommending somewhere to start it'd probably by Rousseau or Macchiavelli, but I don't know how universal (if at all) that opinion is.