"I'd love to play D&D but I don't know how. I don't want to ruin it for everybody."
I've heard the same sentiment from many fans of videogames: they are scared and intimidated by playing tabletop RPGs, even though they really want to play. The fact is that most videogame players already have the tools to be great tabletop gamers. When you pick up a videogame for the first time, there is a learning process as you deduce the underlying systems at play. The same is true of tabletop gaming, but, while it may seem more complicated, it actually can be easier to pick up because you have a support group there to help instead of just the manual.
No one learns how to play a tabletop game from just reading the player's handbook. You have to experience the game in play, and learn by doing. But there are some things to keep in mind that will help you be a better player, things I wish someone had told me before I started playing. I'm going to assume that you've taken the first step and found the right group to play with (that's a column topic in and of itself). Here's what you need to know as you prepare for that first session:
[For those expert tabletop gamers out there, these guidelines will remind you what it's like to be a novice player and, hey, maybe break you of some bad habits.]
Befriend Your GM: Tabletop games call it different things, but the Game Master (a.k.a. Dungeon Master) is the most important person at the table. Before you arrive for your first game, you should reach out to your GM. The GM will guide you in making a character as well as help you craft a background story that fits his campaign or adventure. Whether it's through email, on the phone or in person, a quick discussion or two with the GM will ensure that your character is seamlessly blended into the party. Believe me, the GM will thank you for being a diligent player, and it will pay off in story hooks that will make sure that you character is immediately involved in the story. Listen to his advice and work with him the best you can, and maybe even talk about a few non-game-related topics so that you establish a rapport. You don't have to be besties right out of the gate, but a quick convo about how awesome Clash of the Titans was or how much you hate Twilight never hurts.
Ask Questions: What's that old saying? There are no dumb questions. Well, that's definitely true when you're playing a tabletop game. If you don't understand something, you shouldn't hesitate to ask about it. It's the only way that you will learn. If you don't want to trouble the GM or if he's busy, you can always ask a player next to you. I've never met a gamer who wasn't eager to show his nerd chops by telling you about a specific rule or spell. Take advantage and ask as many questions as you can.
The only caveat here is that you should try to time your questions either when there's a lull in the action, or when the spotlight is on you. Nobody likes to be interrupted in any social situation and most tabletop games are merely structured social situations. See Don't Be A Dick below.