That is not just dirty--it's filthy. Why have you not noticed it before now, and cleaned it regularly? Get some Tilex, pour most of a bottle of it in and let it stand for about an hour. Close the lid, because the smell will be strong and it will be working to loosen the crud that's built up. Get a toilet brush and try to scrub it. You may have to repeat it several times. If that doesn't loosen it, then try a cleanser like Ajax or Barkeeper's Friend. Finally, pour a bottle of straight Coca Cola in it and let that stand, then flush it.