If there are any bare spots on your dogs skin, there could me several reasons for this. It could be a nutritional problem, or parasites or an infection.
If the loss of fur is in a circular pattern, then this is most probably ringworm, which is not a worm but a type of fungus. You can check this if you have a UV light at home, as ringworm will glow under it. However, this is not an effective test so a lack of glow does not mean there is no ringworm.
To remove ringworm, you need to put on gloves, and clip the hair around the area that is infected with scissors. Ringworm is contagious so make sure you don't touch yourself or another pet. After you've clipped, wash the animal's entire body with an antifungal shampoo and let it airdry.
If within a few weeks you don't see improvement, then take the animal to a vet.