My sympathies on the loss of your dog. Many high schoolers don’t realize how interesting they are. You can find meaning in even the most mundane of experiences.
Think about what you could write about that makes you uniquely you…something only you can write and lets AOs learn your values, interests, motivations, etc. And if that essay is about the death of your dog, so be it. Or, maybe it could be about your search for the perfect chocolate chip cookie recipe, or going to high school where your parents work (ugh!) or something of that ilk…the point is you can make anything work.
You might spend some time doing some more creative brainstorming and/or do a google search for ‘college essay brainstorming exercises’ or something like that. The college essay guy site has a number of free exercises geared toward identifying something you might write about as well, including many sample essays. Good luck!