I have very severe pain in my abdominal and doctors have mentioning endometriosis what tests do I need to have done to find out and what exactly is it?

Surgery. The only way to definitively diagnose endometriosis is to have a surgery called laparoscopy. Other tests that can help can be a pelvic exam and ultrasound.
Laparoscopy. Sorry you are having so much pain. First you should have an exam and a pelvic ultrasound to look for anything obvious. If endometriosis is the concern, you can either begin medical treatment or seek a diagnosis. The only way to make a diagnosis is with laparoscopy to take a look inside the belly and get a biopsy sample.