If you can't afford to get some dog toothpaste you can make a safe toothpaste for dogs by mixing some baking soda with some water into a paste consistency.
Mix the water and baking soda until it's made into a paste and then use that baking soda and water toothpaste to brush your dogs teeth.
Never use any human toothpaste though on your dog because it contains fluoride which is deadly to dogs.