How serious is the situation? Is your dog bleeding heavily? Many skin wounds may only need cleaning and a little antiseptic cream. Most wounds stop bleeding by themselves after a short time.
However, as you can imagine, if the wound is bleeding profusely, you must immediately visit your vet. In the meantime, you should take steps to stem the blood flow, like putting pressure on the wound with a clean towel. Or, better still, with a pressure bandage from your first aid kit. These measures can be lifesaving on the way to the vet’s surgery. You can make a pressure bandage with cotton wool and bandages: place a large wad of cotton wool directly over the wound, then bandage it firmly. If the wound is on the leg, apply bandage to the whole leg. Then take the dog to surgery.