My Dearest Player,
You have a wonderful friend. As a DM, he or she has put a lot of time into preparing for the game you're about to play. There’s a good chance that while you’ve been out doing fun stuff, playing video, sportsballing, exercising at the gym, or whatever it is you do for fun, your friend was hunched of a stack of books, paid for out of their own pocket, taking notes, designing a campaign specifically designed to entertain you.
The following is a list of things to keep in mind and do that will go a long way towards not only showing your appreciation, but having the most possible fun in the long run. It will take you just a few minutes to read, and it will make all the difference.
Thank them after each session.
They have put a lot more time into planning this than you realize. Make sure you voice that appreciation and don't take it for granted. If you're frustrated, bored, confused, or otherwise aren't enjoying the time, consider chatting with your DM about your experience. There are very few things in life (and D&D) that can't be solved with good communication.
Be present.
Turn your phone off. The minute you slip your phone out start thumbing through Reddit, a little part of your DM dies, along with a little bit of your campaign.
Just like with sports, your team benefits from everyone paying careful attention to what’s happening, even if it’s not your turn to swing a sword or cast a spell. You can be planning ahead, taking notes, shouting encouragement, role playing, adding flavor and commentary, and making everything more fun.
When you check out, the game slows down. Please be a team player.
Learn your class.
Read up on how to play a Rogue, Druid, or whatever character you rolled. That goes double for spellcasters. The biggest time sink in D&D is players shuffling through lists of spells mid-combat trying to decide what to do. In the first few levels it’s only a handful of spells. Learn what they do and write down scenarios when you’d use them. You can even buy a deck of spell cards to keep them at your finger tips (but don’t rely on those entirely, know your spells.)
Be creative!
Your DM is already expecting things to go off the rails. Don't worry about what you think they want you to do. Instead, think about what your character would do! Just remember that when you throw a curveball, be sure it's not selfish. Make sure it's something everyone can enjoy it.
Use the environment.
Just because you play a melee class doesn’t mean you have to swing the sword mindlessly at goblins, round after round. Ask your DM for details (or subtly suggest them -- your DM probably has a lot going on in their mind) about the room or area you’re in, and look for ways to use it. For example...
Is there chandelier you could cut down? Great! Use your action readying an attack on the rope holding securing it so you can crush a whole group of baddies at once.
Try grappling a monster and then pushing them off a ledge. It's cool and you might be surprised at how much damage you can cause without sinking a blade into something.
Bring snacks.
The DM has earned his or her Doritos with prep time.
Attendance and communication is key
Do your very best to attend all sessions and be on time. If you know you’re going to be late, let them know. This is common curtesy for life in general, but it can really sting extra in this scenario. The DM may feel (whether it's true or not) that you're not enjoying the campaign and may take it personally. DMing isn't easy.
Keep notes!
When your DM mentions the name of some wizard and his tomb of secrets, write it down. When the dwarf says something funny, write it down. When you meet an NPC that gets a name, and it write down along with any other details. Your DM is looking for ways to make the world feel real and is probably using NPCs to do it. If you treat them like decorations they’ll feel like decorations, but if you treat them like real people, you’ll often be pleasantly surprised.
Ask NPCs for gossip and rumors.
The taverns and pubs are a great tool used to connect your adventure to the world you live in. With much your time spent delving the deep, exploring deep dark forest, and scouring ancient mountain ruins, there aren't a lot of non-gimicky ways to bring the world to you.
NPCs in a tavern (or on the road, in a shop, etc) are a great place for your DM slip out small secrets, foreshadow events to come, provide optional side quests, and spice things up. They may not always connect to the immediate story, but once something happens in the world, it can come back around again.
Speak up
Whatever your character’s backstory is, it didn’t happen until you say something out loud about it. That 2 page document about your killed parents, broken past, and life on the run? As far as the other players are concerned, it didn't happen. Don’t play a quiet, lone wolf character. Leave that for other games. D&D is social, and it’s way more fun if you join in the action.
Don’t betray the party.
Don’t pickpocket or steal from party members. There’s no loot so good that you need to make things weird around the table. The DM is going to scale encounters to your party, so you don’t need a better sword to progress. It just means the monsters will get harder. And cooler. I mean, yeah, go get some sweet loot. Just make sure to earn it. Don’t be a jerk.
Give your character a quirk
Little subtle details bring life to your character. Give your character an odd habit, tick, phobia, interest, or have them obsessively collect something strange. It will give your DM a chance to add meaning to it, and make the story richer. Snag those mushrooms. Or goblin ears. Or fireflies. Get creative.
Tell your DM what you like.
If you dig puzzles, make you tell you them after a session containing them. They're trying to figure out what connects with you, and being transparent is the quickest way to maximum fun.
Buy your own copy of the Player’s Handbook.
Your DM probably says it’s fine if you grab his or hers to look up a spell, ability, or rule, but honestly they may really need that on their side of the table for reference. They're just being nice. Passing that one copy around can slow a game down to a drag.
Plus, if you own a copy, you can actually read it, and then you’ll know stuff, and you’ll be even more helpful, and everything will be more fun.
But most importantly...
Just have fun. D&D is a blast.