It was difficult to tell much from the picture, but what you're most likely dealing with is some form of gangrene. This means that for some reason the tissue on your lizard's tail is dying. The two most likely candidates for causes are a small wound that became infected (wet gangrene), or an injury that reduced or completely blocked blood flow to that area of the tail (dry gangrene). In either case, it is vital that you get your lizard to a vet.
First, it is practically impossible to correctly diagnose the problem over the internet.
Second, wet and dry gangrene, and the extent of the necrosis will call for different treatments. And the problem may be something else entirely (ie, a fungal infection).
Third, without the proper treatment your lizard's life may be at risk. With wet gangrene the infection and necrosis will continue to spread if left untreated. Eventually this will result in septicemia and then death. With dry gangrene there is (hopefully) no infection present and the gangrene may stop spreading on it's own, but the affected area will still need to be amputated or will fall off on it's own. This leaves the area open to infection.
So in either case your lizard is in need of medical attention either just for antibiotics, or for amputation and antibiotics (assuming you're dealing with gangrene and not something else).
The suggestion that another poster made that you simply spray the wound with water will at the least have no effect, and at the worst will cause the infection to spread faster. Bacteria thrive in warm moist conditions.
If you are in the United States you can find a herp vet by going to arav.org, herpvetconnection.com or anapsid.org. If this fails just start calling vets in your area and ask who has experience with herps. You'd be surprised at how many vets simply don't advertise their herp experience.
Let me know if you need more help beyond that, and please keep us updated.
-Z