Remember the Answers: You know what? There is such a thing as a dumb question: the same one you just asked five minutes ago. If you can, try not to repeat the same question ad nauseum. For example, I played with one new player who asked what dice to roll every time he was instructed to roll for initiative. After the fourteenth time, I anticipated his question and handed him a 20-sided die when combat started. While most players pick up rules pretty quickly, sometimes there's a mental block or you just can't remember what to do. If that's the case, write down the answer and refer back to it when that situation arises again. Your fellow players will thank you for it.
It's Ok To Make Mistakes: This seems to be a hard concept for players who transition from videogames to grasp. It's understandable for a player to want their character to be the best that they can be, but part of the fun in tabletop gaming is failing spectacularly. The thief accidentally triggering a trap that he could have disarmed, or the wizard who looses a fireball in a closed room, or the fighter charging a medusa with eyes wide open; each of these moments end up hurting the party, sure, but they can result in great stories. The fun of tabletop is playing out these moments: maybe the thief now religiously checks every door and chest for traps, or the party cringes each time the wizard starts casting fireball or the fighter is called Blockhead behind his back.
The point is, some players get hung up on doing the exact "right" thing in any given adventuring situation and end up paralyzed by the risks. I've got news for you: Adventuring isn't a walk in the park. There are risks around every corner and it's ok to sometimes make a bad decision. If another party member tries to warn you, pay attention, but realize that it can be fun to do something "stupid" and reckless. If it works, you're the hero. If it doesn't, hey, at least you have a fun story to tell when someone asks you how your tabletop session went.
Don't Be A Dick: This one is pretty self-explanatory. If you are genuinely interested in the game and have fun with your fellow players, you'll get asked back. If you cause problems, demand that other players do what your character says because "I'm roleplaying an officious dick" (a.k.a. paladin) then chances are the next session will go on without you at the table. One way to make sure you're invited back: bring snacks. It's just not a gaming session without Cheetos.
Have Fun: The last thing to remember is to just have fun. Joking around the table is not only okay, it's preferred. As you adventure in whatever world the game is set, there's no reason not to enjoy yourself along the way.
Playing tabletop RPGs can be an extremely rewarding experience because you have the opportunity to create a story cooperatively with your fellow players and the GM. Every person at the table is important, from the expert to the first timer. Your presence brings something unique to the table; it would be a different experience if you weren't there. And if you follow the guidelines above, you should have no trouble making that experience a memorable one.
If you're still nervous about going to your first tabletop session, here's a consoling thought that a GM once told me: every tabletop gamer started out just like you. So don't be nervous, and get out there start rolling those dice.
Greg Tito checks for traps every time he walks into the office, just to stay frosty.