How Do I Prepare To Move Out?

I'm a full-time college student and I live with me two triplet brothers and mom. From my previous posts, the middle brother is mentally ill and also takes on the arrogance of our narcissistic father that moved out. This brother of mine just showed his knife to mom before attempting to "make peaceful conversation" with her. She refuses to continue with the eviction process and I'm done playing bodyguard. How do I begin to narrow down what I have to only things I need and save up enough money to move out?