Here is a website with information about earthworms - Facts about Earthworms, and this one tells you what earthworms eat
Here are some videos you can watch - Worms are Wonderful and Worms
Now that you have all your facts about earthworms, talk about what you have learnt with someone else. This will help you form your facts into sentences for your story. Think about the order you will write your facts.
What will your opening sentence be to hook the reader into your story Maybe you could use a question to hook your reader, for example
Did you know earthworms are in your garden?
Have you ever seen an earthworm?
You might write your story all in one day, but you can work on your story over two days. Draw a picture of an earthworm to go with your story.
I wonder if you can find an earthworm in your garden?