Say what you mean to say. One of the benefits of learning online is being able to think about discussion contributions before sharing them instead of having to contribute "on the fly." Take advantage of this to reread your discussion postings before you submit them (it helps if you do it out loud) to ensure you're saying what you intend to say. Consider where you may need to define or restate words to clarify understanding.
It is likely important to pay special attention to using figurative language. It's possible not everyone will understand what you intend, especially those for whom English is not their first language. It is likely helpful to be mindful of cultural differences in choosing if and how to use figurative language and idioms (such as "It's raining cats and dogs"), and to consider including additional explanations if you do.
Additional cues may also be helpful to convey the nuances of joking and humor. A clarifying comment or parenthetical such as, "hee hee," "I jest," or "my tongue is firmly planted in my cheek when I say this" may help others read your comments in the tone you intend. If someone misunderstands what you have said, taking responsibility for clarifying what you meant and, when appropriate, apologizing, can go a long way in earning and keeping good will in a discussion.