You will need to make a character and can find more information on how to do so on this page.
Be sure to read that page thoroughly, almost all the info on it is important for making your own ninja.
To join a faction look at your options here before talking to an admin on the subreddit about joining one!
As you role play and your character develops, visit this page for simple instructions on how to improve your character by adding to your stats and gaining new abilities.
Once you have made a character, you can submit them for approval.