Can anyone help me with information I might need for the membership application?

My father‘s side has Choctaw lineage. I no longer have contact with him. Before my mom’s side went no contact with him, he got my older sister enrolled. I know the information of my grandparents, but thats about as far as I am able to trace back. I’m going to start applying to law school soon and would like to have membership before then. Can anyone help me with potential information or documents I might need?