My dog hit its head, is foaming blood, its nose is bleeding, and it’s drooling — what should I do?

Nose bleeding after a head bump may be damaged nasal mucosa, but with blood in the foam too, concussion or brain injury can't be ruled out. Go to a vet promptly. Before that, do basic first aid: clean the blood, and ice the bridge of the nose to constrict vessels and help stop the bleeding.