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What should I discuss with my DM prior to my first game? Classes, races and sub-races? There is a lot of info in multiple sources; any word on what to discuss with DM? I'm new to D&D.

Everything.

In no particular order, you'll want to discuss:

• 
  Campaign style and tone. What are the expected PC actions? Who are the major factions, and what are they like? What kinds of acts are "too much"? What kinds of characters are acceptable in that style? If it's a grim and serious game about dark choices then playing a bubble-headed free-spirit that tries to make everyone smile would violate the style and tone of the campaign; potentially in unacceptable and fun-destroying ways.


• 
  Allowed source material. Not all DMs allow all published sources; they may not allow all material from a particular source (such as Xanathar's or Volo's). Trying to play that material is going to cause problems. Avoid those problems by discussing what you want to play, and what the DM allows, before the game starts.


• 
  Table conventions. What resources is each player expected to bring? What rules govern the gaming space? How do dice that land oddly, or on the floor, get handled? What's the snack, food, and drink situation? These can be a big deal and can lead to spectacular arguments if someone violates the rules (in ignorance or in malice). These rarely change between campaigns for the same group, but you want to learn them every time you join (or form) a new group.


• 
  Player quirks. Everyone has issues, lines, triggers, or whatever you want to call them. When those get crossed, players react in extreme ways that can include: crying, violence, screaming, destruction of property, suicidal behavior, etc. Ask your DM about any such quirks your new group may have, so you can avoid them. Example: A friend of mine cannot handle zombies, so when she plays I disallow necromancer player characters and minimize the presence of reanimated corpses in my foes; she's a close friend and any prospective player that can't respect that line can go find another table.


• 
  Party composition. While every character can be interesting, D&D is a group game that benefits from teamwork. Ask about what the current group is like and what roles they cover. You don't have to fill their gaps but you need to be aware of those gaps when designing your character.


• 
  Character optimization. Different campaigns expect different levels of character optimization. Learn how important it is for your game by talking to your DM. If your build is too optimal then you'll make encounters much easier and may make other players feel superfluous and reduce their fun (that really sucks, as I can attest, and is why I no longer allow players to roll dice for character stats). If your build is too sub-optimal then you'll feel like a weakling (probably having less fun) and are likely to stress your fellow players if they feel obligated to help your character survive (reducing their fun).


• 
  Setting details. At a minimum you need to know the general style and era of the setting, as well as how common magic is and what the usual reaction is. If you can remember more then ask about cultures, politics, religion, economics, or whatever features the DM thinks are relevant. If you can't remember more (and I've had that problem a few times) then make it clear that you care but need to process what you've already learned and will ask later.

Welcome to D&D! Good luck!