Student:
Join the Academy. [Therefore; the roleplay/forum....]
Genin:
Pass the academy final exam. [Basically, fill out a description of your bio...]
Chuunin:
Pass Chuunin Exam.
Jounin:
Pass Jounin Exams.
Ninja Councilor:
Your ranking must be Jounin or higher.
Sannin:
Appointed by Kage, or Hokage. [More then likely the decisions of admin/mods]
Kage:
Appointed by Council [Mostly by the decisions of all admins/mods]
Medical Ninja:
Medical Training Camp. [If you're already skilled, then it's automatic]
ANBU:
ANBU Training Camp. [Normally, the admins pick the best for the job]
Hunter Ninja:
Hunter Training Program.
Missing Ninja (S-Ranked Criminal):
Piss of your village leaders! [Or, even if you try to attempt to kill your own shinobi of your village.. Or just ask the admin or one of the mods and they'll tell you...]