1. Know the rules of the game you are playing. You don’t have to memorize every book, but you should at the very least be able to roll up a character and understand the basic dice mechanics of the game.
2. Know how to make a decent character, know what constitutes a good character. Building a D&D character that averages less than 1 point of damage per turn is a useless character, on the other end of that scale, a Call of Cthulhu character whose primary skill is shooting/punching things, is likewise useless.
3. Know your group. Try to get to know everyone in the group, try to find out what they want out of the game and what their strengths and weaknesses are. If you are not a good fit for the group, politely drop out of the game.
4. Know your GM, know what kind of game he wants to run. Find out what his expectations are, does he like a fast paced shoot em up game, or does he prefer a slow burn plot. Build characters that play to your GM’s strengths, don’t build characters that exploit or highlight the GM’s weaknesses.
5. Be a player that other players like to be around, play characters other characters want to be around. If you find yourself uttering the words “I am just playing my character!”, you have failed this one.