While I generally try to believe that humans are more good than bad and try to help each other when they can, you only have to look out of your car window on any trip to realize the devastating effects humans are having on the Earth. I'm old enough to see the great effects humans have had on our local environment, and it's not a positive influence. I think this is the easier position to write from, and there is plenty of evidence to support the negative ways humans impact the environment. I think one of the most harmful ways we destroy our planet is through pollution, so I might begin with a thesis along these lines:
Humans negatively impact the environment through pollution of the earth's land, water, and air.
Now your three body paragraphs are made clear, and you just need to find the support for each of those paragraphs.
In the paragraph about polluting the land, you might want to research how many pounds of waste the typical person or family produces per year. How many pounds of waste end up in landfills across America? What percentage of people recycle their waste? (It's not nearly as high as you might think.) What effect is all this waste having on our soil and land? You might also look at the ways technology specifically is impacting our earth. All those computers, cell phones, dishwashers, dryers, and refrigerators end up in landfills somewhere, taking up lots of space and leaking potentially deadly components into our soil and underground water systems.
We've all heard about how plastic straws are harming sea turtles, but the pollution of our planet's water is much more complex than this issue which has gained attention. If you've ever been in a boat on the open water, you've no doubt seen bottles, plastic wrappers, or even Styrofoam coolers littering the water's surface and edges. You have to wonder how much more sits on the bottom of lakes and oceans. Humans also dump deadly chemicals into our water sources, creating toxic environments for the aquatic life which depends on clean water. You may even be shocked to learn that businesses are often allowed to legally dump chemicals into lakes—and can often pay money to the state to be allowed to increase those amounts. We recently had a sewer line break near our community, releasing millions of gallons of sewage into our local lake. Clearly this is dangerous for humans, but this type of scenario is more common than you might think and is often kept quiet. Research to find what types of water contamination might be prevalent in your own community. Your local Department of Health should be able to help—or could even help you test water purity from local sources of water.
I'm from the East Coast, so when I visited LA a couple of years ago for the first time, I was completely shocked at the amount of smog hanging in the atmosphere. Air pollution is dangerous for humans and animals, and it creates all sorts of health problems. Again, depending on where you live, industries are often allowed to legally release various amounts of pollutants into the air. Some of these pollutants have proven to be carcinogenic, and some people are more sensitive than others to the effects. What air pollutants are most common, and how do they impact humans and our ecosystems?
You might want to conclude your paper with some suggestions for change that could help better protect this one amazing planet we've been given. I would argue that we all need to make increasingly better choices to help create the positive impact needed for our struggling environment.