First
Does your pet have everything it needs to live a long healthy happy life? Is your pet comfortable and clean? Some pets are much more sensitive to their surroundings than others. For example: Does your lizard have the proper lighting and heat sources? Is your fish tank dirty? Is your dog itching more than usual?
Second
Is your pet getting a proper diet? Diet is a huge part of their survival. You wouldn't feed crickets to a big python. You wouldn't feed Twinkies to your dog. A sick pet can get healthy with a better diet.
Third
Unfortunate things happen. Educate yourself on how to tell if your pet is sick or hurt. There are simple things such as vitamins, supplements that your pet is in much need or is deficient of. There are some over-the-counter medications that you can get but be sure to educate yourself about such things before administering. Some injuries may just require a bandage and a careful watchful eye.
Explore more expert advice on caring for dogs and cats at this resource.
Lastly, you may want to just take your pet to the local veterinarian if you're not sure what is wrong with your little friend.