I'd give the following advice to a newer player, even if some parts may seem irrelevant, because certain scenarios will likely arise during their time on the Discovery server. This is not necessarily "common sense"; some players might see it as overly conservative, but I think it's important to build a good attitude from the start.
1) Know the difference between ooRP and iRP. Be able to separate the two when playing. If you're unable or unwilling to acknowledge the distinction and act appropriately, you shouldn't be playing on a roleplay server.
2) Treat every encounter or conversation as being in-character unless it's abundantly clear that it's out-of-character.
3) Never try to dictate the rules to another player. In attempting to explain them or prove you're correct, you're liable to misinterpret or get them wrong yourself. If you're convinced they're breaking the rules, take screenshots and move along, then file a report on the forums. There is no need to be a vigilante. If you must discuss the rules, suggest politely by //oorp private message that they check the relevant sections of the forum.
4) Don't metagame (use knowledge that your character could not have acquired on their own) or powergame (act in a manner that does not permit others to respond appropriately, thus forcing a particular result).
5) Be forgiving. The entire population of the server isn't out to make your life a living hell (unless you're really asking for it). Try and be considerate of what the other players you're interacting with are experiencing.
