1) Google is the most important website. I find myself using Google constantly when talking to people, not only when I need more information about something, but when other people need information about something and are too fucking lazy to go to Google, and end up asking me where something is instead. God damn.
2) Need a quick run-down on what something is? DON'T ASK ME! Use Wikipedia. It is your online encyclopedia. It's probably where you should be contributing a big chunk of your own knowledge, assuming you have any.
3) This one is obvious: be literate. Actually, this is optional if you intend to be nothing more than a lurker, but if you ever, EVER plan to talk to ANYONE who would call themselves an Internet citizen, PLEASE act as if you're writing to someone else, instead of typing shit reminiscent of shorthand courtroom notes. Capitalization is not important. Perfect spelling isn't required. But people will always judge you based on your appearance, even if your appearance is nothing more than a few shitty acronyms and smilies.
4) E/N. Important phrase to learn. It stands for "everything/nothing." That means, something might mean everything to you, but it probably means nothing to everyone else. There are plenty people on the Internet, at least a few dozen, and I'd be surprised if a great number of them would want to hear what you're doing. That's why forums, and even INSTANT MESSAGING are NOT appropriate places to share the details of your day. You don't speak about yourself unless spoken to. This is what a LiveJournal is - a personal area to put out whatever opinions you might have. I'm sorry if it feels confined and conformist, but it's for your own protection.
That's about it! Welcome aboard. Anyone else have some crucial rules I might have forgotten? It's hard to list things like these because to ME, they're common sense.