Every GM (game master) has their own expectations of how their players should conduct themselves. Some RPG's have very explicit rules on how you join, what characters you can join as, and how active you are to be in the RPG. Others have very few rules to govern the players.
Before you join, make sure to search around the website for all of the information on the RPG. Read the rules of the message board itself as well as the rules for the RPG. Most GM's will create a FAQ for their RPG's, so be sure to read that, too. After you've read everything, you should contact the GM/webmaster if you have any questions or confusion. It's better to have all that settled before you join.
If you have any complaints or comments about the rules, be mature and discuss them personally with the GM in a polite manner. If you have good reasoning behind your complaint, a good GM will be happy to try and fix the problem. Do not join the RPG with plans of breaking the rules you do not like. It will only get you suspended from the game and make you, the GM, and the other players unhappy. When you join an RPG, you are binding yourself to obeying the rules. If you can't discuss the issue maturely, then deal with the rules or do not join the RPG.Subsection I.II: Conduct
Always be sure to use good sportsmanship while playing. Do not argue with other players on the message board. Personal issues should be kept on a personal basis and therefor taken to e-mail or any other private messages. If a player is breaking rules, do not take it upon yourself to handle the situation. Moderators and the GM are there for a reason. If a moderator or the GM does not notice the rule-breaker, bring it to their attention either by e-mail or by posting a message on a designated board outside the RPG. God-moding is a form of bad sprtmanship. Be aware of what it is and avoid it.
Above all else, have respect for your fellow players. This very important, as you will not get respect if you do not give it. Since RPGs require the interaction of the characters in order to make a story, it is necessary that the players behind the characters can get along. Never degrade or insult a player for making a mistake. Do not point them out as "bad role-players". No one was ever a perfect role-player from the beginning. You should respect that everyone needs a place to begin and learn.