Since you already have some understanding of the form, it sounds like you need someone to brainstorm with a little. Many people do not believe they are creative, but find they can be if they are enthusiastic about a particular topic.
You mention pets and the earth as possible subjects for your sonnet, and those are both fine, if you like those ideas. But those are very broad ideas, and that might be what is hampering your creativity. Try narrowing either of them a little.
For example, do you have a pet you could write a sonnet about? I have a black Lab, and I know I could easily write a sonnet about her. I would start out with a list of words about her and build from there. If you do not have a pet, you might want to consider writing a sonnet about a particular kind of animal, maybe cats or dogs or even iguanas. (We have one of those, too.) The idea is to keep it narrow. When your topic is a manageable size, it is much easier to be creative.
The earth, too, is a very big topic. How could you narrow that? Perhaps you could focus on its preservation, something we are all worried about. Or perhaps you could focus on a specific aspect of earth, maybe its oceans or its mountains. Do you have a favorite spot that you could describe in a sonnet?
A sonnet can be written about a person, too, you know. It could be a girlfriend, a friend, or even a parent. Maybe there is someone in your life you could write about. Think how pleased that person would be.
Once you narrow your focus, think about the descriptive details that you want to include. Make a list of words that come to mind when you think about your subject. I know you can be creative if it is a topic you care about.