A good thesis statement needs to mention your topic and make a strong statement that you will later build upon in the body of your writing.
Something along the lines of, "The reason cats are superior to dogs is because..."
Now, you'll want to have several reasons why cats are better than dogs to add in there. These will be a matter of opinion (not factual), but YOU are making the argument so you state them as if they are factual.
What could they be? Cats are less maintenance to take care of, they live longer, etc..