First degree burns are what generally results when someone touches a hot iron or a hot stove and they only affect the top layer of skin. Sunburn is a form of first degree burn as well.
Second and third degree burns are more serious and require immediate medical attention.
Also, if the first-degree burn is very large, wraps around an extremity, is located on the face or genitals, or if the victim is an infant or elderly person; you should go to an emergency room immediately.
Most first degree burns can be treated at home with basic first aid but it's still important to know what to do; and what not to do.
To treat a burn, dermatologists recommend to:
Cool the burn. Immediately immerse the burn in cold tap water or apply an ice pack. Do this for about 10 minutes or until the pain subsides.
Apply petroleum jelly 3-4 times daily. Do not apply ointments, toothpaste, or butter to the burn; as this may increase the risk of infection. Do not apply topical antibiotics either.
Cover the burn with a non-stick sterile bandage. If blisters form, allow them to heal on their own. Do not pop them.
Take over the counter pain medication. Acetaminophen or Ibuprofen will relieve pain and sooth inflammation.
Stay out of the sun. Once the wound heals, keep it covered or stay in the shade. You should also apply a broad-spectrum, water-resistant sunscreen with an SPF of 30 or higher. This will help minimize scarring and redness, especially in people with darker skin tones.
Sources:
American Academy of Dermatology: "How to Treat a First-Degree, Minor Burn"
Very Well Health: "How to Treat a Burn"