The word "decalogue" means ten commandments, so this question is asking you about Finny's character. Do you understand who he is if you were able to list ten things that he believes in doing. There isn't a clear cut list of ten things, but you can see through his personality and the things he says and does to determine what those ten things are. Depending on your assignment, you may need to quote Finny to support what you claim; but for this answer, let's just mention a few. Finny is athletic - Thou shalt always play a sport.
Finny is competitive - Thou shalt always win!
Finny is playful - Thou shalt always play fun and interesting games.
Finny is sociable- Thou shalt include everyone in the game.
Finny is loyal - Stay true to Gene as a friend.
Finny is merciful - Forgive Gene no matter what.
Finny is never serious - Never be serious.
Whatever you decide to include in Finny's list, remember to follow the assignment up with evidence from the text as dictated by the instructor.