Nerds enjoy displaying their knowledge, the more obscure, the better. This is why they like Trivial Pursuit so much. For many nerds, one of the primary methods of displaying their obscure knowledge is by referencing their cultural touchstones in every day conversation. In order to effectively communicate with them, it's helpful to have a grounding in those same cultural touchstones.
Of course, being individuals, each nerd you encounter will have a slightly different list, and some will disagree with any element with almost religious vehemence. I'll divide my list into several categories: Movies, Television, Science Fiction, Other Fiction and Non-Fiction. (I've yet to find a unified genre of music, but Jonathan Coulton and Wierd Al seem to cross boundaries.) Keep in mind, this is intended to serve as an introduction to Nerdish, and is not (by any means) as comprehensive list.
Movies
Movies are watched obsessively. A nerd doesn't count a movie as great unless it's something they've seen at least a dozen times. If you've memorized the whole movie, all the better. Start with these: Blade Runner, TRON, Real Genius, The Fifth Element, and either the Star Wars or Star Trek series. Nerds may enjoy both, but for most, one will be mere entertainment, while the other is canon.
Television
In keeping with the obsessive focus that nerds maintain, they tend to watch every episode of a series, frequently multiple times. It's recommended that to begin immersion in nerd culture you watch at least one episode of each, and then choose one series to watch in its entirety: Star Trek (any of the various series), Farscape, Buffy the Vampire Slayer, Battlestar Galactica, Doctor Who, Quantum Leap and Babylon 5.
Science Fiction and Fantasy
Nerds read science fiction and fantasy. It may be the single most common thread for nerd culture. Usually, in addition to reading the canonnical works, they will fixate on a particular author or authors and read every scrap of writing they can obtain by that person. The canonical works include Dune, Neruomancer, Cryptonomicon, Sandman, the Discworld series, Stranger in a Strange Land, The Martian Chronicles, and the Foundation series.
Non-Fiction
Nerds like to specialize. The more you know about a very narrow topic, the more likely you are to have a piece of knowledge that no one else in a situation has, thereby allowing you to display your intellectual prowess. The subject matter of choice is actually less important than the depth of knowledge on has on that subject. Because of this, it is difficult to choose a specific subject to study when attempting to become more conversant with nerds. Fortunately, the fact that you will likely not have the depth of knowledge of a given field that the nerd you are conversing with does, is exactly what they want. So, to get a grasp of the kind of depth that nerds aspire to, read The Story of Light, by Ben Bova. It is a fine example of the how a single topic can be used as the basis of a vast body of knowledge.