Whatever you were learning, write about it. Since this is post is in middle school, your teacher probably expects atleast half a page to be written in. Write about whatever was asked to be written in the entry. If you're reading a book for class, it may need to be a summary of what you read in the book. If you had an assignment, it's probably about the assignment. "Journal entry" means to write in your notebook, so this wasn't very clear what you need help with, ask a classmate about what assignment needs to be written about
