Okay as you are aware when signing up you choose ranks E-A this however does not reflect your standing within your village. Below is a list of ranks available in each village, you must meet each requirement before applying for that Rank.
Genin: E-D
Chunin: D-C
Jounin: C-B
ANBU: B-A
Sannin: A-S
Kage: S-SS
Obviously the last two cannot be chosen to begin with, you must work to attain these titles, training your stats ect doing great Rp's with your character on the forum. When you have attained enough stat points and the staff feel your character is deserving you will be granted special titles to reflect your characters standing on the forum. (Eg. Sannin)