If you want to go for the position than you are allowed to challenge the current captain to the position, though they don't have to accept the challenge. If they do then a topic is set up in the off-RPG area between you and the captain, where you will both fight for the position, showing your skill in writing and in fighting.
An admin will monitor the fight, reading every single post, and at the end they will determine who the better fighter was, sometimes regardless of who actually 'won' the fight; eg, if person A won the fight, but person B was a far better RPer, than they could still possibly win the position over the 'winner'.
If one person is unhappy with the result then they are allowed to ask another admin for a different opinion, but if they have the same answer than the decision stays.
So if you want to follow through with this just PM the current 12th division captain to challenge him, and if he accepts just PM an admin to let them know that this fight is going to be taking place.