1) Before creating your character you must post a sample RP and wait for it to be graded. This will determine how powerful your character can be. grades will be as follows:
D: Allows a player to make a character at the Academy student or Genin level
C: Allows a player to make a character at the Chunin or Tokebetsu Jounin (Special Jounin - Jounin level in one area but Chunin in all others.)
B: Allows a player to make a character at the Jounin or Anbu level
A: Allows a player to make a character at the Medical Ninja or Missing Ninja level (Missing-nin and medic-nin can technically be any level ninja, but this make things less complicated.)
S: Allows a player to make a character at the Kage or Sannin level.
2) When posting your sample try not to control NPC characters. This shows that your RP skills are low in situations when there are not many people around and controlling NPC characters will lower your grade.
3) Remember to use proper spelling and grammar when posting. Poor spelling and grammar will result in a slightly lower grade.
4) Para RPing will lower your grade more then anything. Give sufficient detail but don't go overboard. Giving to much information can confuse people who are trying to read you post, and its just plain annoying.
5) RP samples that are not combat type RP samples will get the best grades as it shows that you can be creative when your not fighting. This leads me to believe that you will post well in fights as well, making me want to give you a better grade.