The point of the game is to have fun together and make up a nice story. Think of that when making your decisions.
You only control your own character's actions, not the effects, much less other character's actions. Exception: the game master describes the environment and the NPC actions, unless otherwise noted.
When your character is under attack, you decide whether he's been hit, and how badly. But be reasonable! An invincible character ruins the game, and everyone else's fun with it.
In case of dispute, let the best roleplayer win. If that position is disputed as well, discuss. See also rule one. The GM has the last word.
Play only on your turn. Everyone is entitled to equal participation. OTOH, if you're going to be late by more than 24 hours, let the others know. Leave instructions.
Describe the actions (and/or thoughts) of your character in 3rd person past tense, as if writing a book.
Feel free to ask or give clarifications at any time. Mark out-of-character remarks like that:
It's easier to ask for forgiveness that it is to ask for permission. If you want to add something new into the adventure, go ahead! At worst, the GM will ask you to retract it.
Last but not least...
The rules above are not set in stone (except rule one). Most of them have imposed themselves during actual games. Any exceptions must be approved by the players, though.