So I'd like to offer a few dos and don'ts in playing Dungeons & Dragons. Most people play at someone's home, but even if you play at some other venue, most of these still apply.
Inspired by my gaming group.
Arriving for the game
Arrive no more than 15 minutes early. If the game is scheduled for 7PM the host is not expecting to have to deal with people at 6:15. People who go to school or have jobs usually get home not too long before game time and may still be setting up, relaxing a little, having dinner, getting dressed, etc. When you show up early you disrupt whatever routine they have before the game and that's rude. The host may not necessarily call you on it, either. They may feel guilty, like saying something to you about it is being rude or inhospitable. Know why? Because they're relying on you, the guest, to know proper etiquette and it's awkward to have to explain it to people, especially adults. When you breach etiquette it tends to make people second guess themselves because they'd rather not believe you're just being an inconsiderate jerk. Just don't do it.
If you're going to be late, let someone know. Ideally you would contact the DM to let him/her know but in a pinch it's fine to tell another player who can relay the message. This is so that the DM can make a decision on whether to start without you or wait. Sometimes the adventuring party are right about to confront a particularly difficult or dangerous foe, and will want to wait for you before engaging so that the battle will be fought with the group at full strength. Sometimes it's okay to just go on without you and you can jump in when you arrive. Only the DM knows the scenario enough to make that call.
Everyone occasionally forgets supplies. Pencils, dice, paper, miniatures, rulebooks, etc. are all things that can easily get left at home if we're in a rush to get ready to go to the game. There's nothing wrong with borrowing spares form someone else in these cases. That said, if you're consistently having to borrow these things from others, you're being a jerk. Sometimes people are extraordinarily nice and bring extra to lend as needed, but if you're relying on that because you either don't care to bother bringing your own or don't want to obtain your own, then you're taking advantage and you need to get your own stuff.
Bring chips, soda, cookies, etc. If everyone pitches in, there might just be enough for everyone. Don't be "that person" who never brings anything. Yes, people do notice even if they don't choose to say anything.
The time before the game starts is the time for socializing, chatting, sharing, etc. This is usually done before everyone has arrived. Don't wait until an hour into the game to share your work stories or talk about how sick you were last week. People came to play D&D, not hear your stories.
During the game
Keep the side conversations to a minimum. The time for those is the pre-game.
Also, yes, we've all seen The Princess Bride, Monty Python and the Holy Grail and Robin Hood: Men in Tights. Yes, those movies are very funny and yes, they're highly quotable. No, it's not okay to quote them during the game at every possible opportunity. That stuff gets real old real fast, and nobody's laughing anymore the 1,456th time the party approaches a castle and someone says "It's only a model." If you think "nobody in my group is really that bad with the quotes" then it's probably you.
The same goes for quips in general. Yes, I'm sure you're very funny and would probably have them rolling in the aisles if you were a stand-up comic, but it gets really old really fast when the DM is trying to establish a certain mood or theme and here you are tossing out one liners like Hawkeye Pierce in the O.R. Sometimes they're great and a useful way to break tension at the right moment, but chances are, your comedic timing isn't that good so it's better to keep quiet.
Don't argue with the DM. Seriously. Occasionally the DM might make a mistake and it's appropriate to point it out, but if the DM doesn't change their mind, it may be that there's more going on than you realize, and your arguing is more likely to mess up the story than improve the game. If you really feel the need to have a discussion then wait until after the game.
Keep your cell phone or other devices put away unless your character sheet is stored in it or something. Nothing is more irritating to the DM (and the other players) than it being your turn to take your action and you have to be brought up to speed on what's happening because you've been texting or checking Facebook for the last 5 minutes. If you have something better to do than pay attention during the game, then go do it somewhere else.
After the game
Help clean up. At the very least make sure you've collected up your own trash and thrown it away. Don't leave behind your empty soda cans, potato chip bags, cookie tins, etc. for the host to clean up later. Yes, that also means keeping track of your trash during the game.
Leave. D&D games played in the evening tend to run late into the night and people are tired. When the game ends, don't stand there and chitchat for an hour. Have you noticed that some of the players have already left? Yeah, your host may not say anything, but they're waiting for you to leave too. See that's the thing. You can go whenever you want. If you're tired you can choose to dart out the door whenever you want to. The hosts don't have that luxury. They're in host mode until the last player leaves, whenever that is. Don't be the last one out the door unless the rest of the group is right in front of you. If you're chatting and notice the host yawning or looking conspicuously sleepy, THAT IS A HINT. Go home. Here's a handy rule of thumb. After the game if you're talking about something or telling someone a story, get to the point in reasonable time, and once you've made that point, don't initiate any new topics of discussion. Save it for later.
"He who takes offense when no offense is intended is a fool, and he who takes offense when offense is intended is a greater fool."
"Don't take refuge in the false security of consensus."