Naval Academy nomination letter?

I understand that in order to apply for the Naval Academy I need a nomination from a congressman. I have two questions regarding the letter that I will send to request one. First, when should I begin constructing my letter and when should it be finished? Second, What should the letter contain? I am confused as to whether it should be a formal letter from me or if there are some sort of guidelines that I should follow. Do I include some sort of resume? How long should the letter be? I have also heard that the congressman that you write will send you everything you need to apply for nomination. I am so confused about the letter that I don't even know if anything in this question makes sense. But I really really really really really want to go to the Naval Academy and an in-depth answer would really help. Thank you!!