A bump on your dog's head could be the result of numerous problems. Without looking at the nature of the bump, it is very hard to make an accurate diagnosis, though.
My only advice would be to take your dog to a veterinary practice. They will be able to perform a thorough inspection of the bump on your pet's head, and hopefully start treatment for the problem right away.
What has caused this bump on my dog's head? The most likely causes of this bump would be:
Trauma: You dog may have suffered a blow to the head, which has caused the tissue to swell up into a bump. Your vet will be able to medicate your dog, to relieve some of the pain and reduce the swelling.
Fungal infection: Most infections, but especially fungal infections, can cause bumps to appear on a dog's skin. Your vet will be able to treat your pet for a multitude of fungal infections.