It depends how deep is the scratch. A superficial scratch such as when a dog is playful shouldn't stay for more than a couple of days, and its generally not harmful.
If the dog intended to cause harm by reaching to you with its paws, and the scratch is serious, you should at least go to a pharmacist. Hopefully he'll give you good advice, as to what dressings to use, or what medicines you might need. If you're bleeding from it you should go to the emergency room.
A lethal scratch from a dog is rare, most dogs bite in that case you might need to ask another question about the bite if you wish.
depends on the scrach