What natural things can I use for endometriosis bad pain at the start of. My period?

Sadly not much. Given that endometriosis pain is basically caused by the inflammation that is associated with active endometriosis, either antiinflammatory medications or hormone based medications are the two things that can help with the pain - they don't get rid of the endometriosis. This means that any natural antiinflammatory or phytoestrogens might theoretically help but nothing has been proven to do so.
No natural treatment. Endometriosis is a serious problem in which little pieces of tissue (similar to the lining of the uterus that bleeds and sheds each month) are trapped inside one's pelvis and abdomen. With each menstrual period, the trapped tissue sheds and bleeds on the inside of the abdomen (the blood has no way to leave the body), causing much pain and cramping. An ob-gyn doctor can treat endometriosis.