What causes very very heavy menstration and irregular periods. I never start when im supose to I have passed blood clots the size of golf balls?

Not ovulating. In a woman with a normal uterus (no fibroids), irregular bleeding is usually due to not ovulating regularly. This can be corrected with birth control pills, weight loss or correcting any underlying problem (low thyroid function).
Contraception. Agree with Dr. G. In addition to her advice, take oral iron and multivitamin supplements to avoid anemia.
You doctor can prescribe birth control pill that limit you period to once a year.