Choctaw Nation of Oklahoma

am trying to trake my liniage to the choctaw. I have researched on Ancestry.com, but do not really understyand what to look for to link my family to the nation. I called The Choctaw Nation and was told to find the CDIB. What? Where is that located? I was also told to go back to 1899-1906 in my tree to find the native american line. Well I did that, but do not know what to do from there. Where will I find the link to the tribe? Would it be on my great grandfathers birth certificate? HELP!!