You can start by developing a basic course outline - such as a curriculum. I usually begin my courses with a basic introduction to the science topic, such as 'What is environmental science?' or, 'a brief overview of environmental science.'
From there, you can start making more detailed sections, such as: 'basic concepts in environmental science', or, the 'many branches of environmental science.' As you move deeper into your course, you become more specific and also use some case-studies to demonstrate your topic more clearly.
For example, I create biotechnology courses; so, I would focus ONE course on a SINGLE aspect of biotechnology, such as microbial biotechnology, or plant biotechnology. try to avoid packing every little detail into one course, it's simply too much and most students will not enrol in a science course that goes for longer than 1.5 hours.
Here are some course ideas for you:
If you want to create a course on environmental science, you don't need to cover everything, because you won't be able to make one single course about environmental science. It's a very big discipline. Instead, focus one single course on a small part of environmental science.
Teaching science can be tricky, however you should definitely start your course. Start small, focus your course on a single aspect of environmental science, and start making your slides. Then, write a script for each slide. Don't forget to write down the course goals. Students want to feel that they've accomplished something, so make each lecture no longer than 5 minutes. In today's age, most people/students want fast results. So, shorter time to reach a goal.
I hope this has helped you. I myself teach science, and I know it's difficult where to start; however, find a niche and go in that direction.