Wanting to move to Germany :) (Berlin)

Hi! My name is Cassie, My brother lives in Germany off base, and I wanted to go live with him for longer than the 3 months, I want to do everything correctly, If there is anyone that can help me or point me in the right direction it would be very much appriciated. I already have my passport. I know I need to prove I live with him. Not very sure what to do after that though or what else I need :)