Academy Student There is only really one requirement for this rank. Weak Character. Meaning by this, if you are a Academy Student, you are going to be very weak. You can't expect to be the lowest rank and beat say a anbu member. lol You get the picture.
Genin For this rank, there is a lot of possibilities but if you have watched Naruto before(and I severely hope you have) then you know that it is all about skill from here on. You can be strong but you will have limits. For instance, you can't hope to be able to stand much of a chance against say a Anbu ranked nin.
Chunnin This rank has to be earned. The requirements for this rank, you will have to post an rp sample(at least 300 words or more), a completed Chunnin exam (which you will have to wait until they are arranged rather than just posting a made up exam) and you will have to have made at least ten posts on the forum.
Junin This rank has to be earned. The requirements for this rank include, a rp sample(500 words or more), 5 successful B rank missions, a sparring match with a Junin ranked member or higher(this is who will be granting the rank of Junin on the person).
Anbu Black Ops This rank has to be earned. Unlike the other ranks, you have to be asked to join the Anbu Black ops rather than you asking. In order to be considered for the position, you have to have completed ten A rank missions and at least one S rank mission.
KageThis rank has to be earned. In order to earn this rank, you have to have completed 20 S rank missions. You have to post a RP sample (1000 words or more) in the Sparring Field, and you have to challenge the current Kage of your village and beat him or her in a sparring match.