Not sure what to write?
First, double-check what the instructions are from your professor. They may have specific ideas or requirements for the discussion. For example, if you need to respond to two classmates, make sure you don't just reply to one! You may also see a rubric, which directly indicates what the professor will be grading you on (clarity, connection to course materials, etc.).
Keep in mind that your classmates deserve respect. Be mindful of what you post!
For your main response, read and re-read expectations and go from there.
If you are reflecting on course content, consider...
The key to a good discussion is to make sure that you are thoughtfully engaging with the text.
You can also look for sample posts and replies, such as this document of sample discussions from Colorado State University.