Just try to set a good example. Name calling is not okay. If you disagree with something, be careful to argue against the idea, and don’t attack the person who posted it. (If it’s impossible to disagree with a post without attacking OP personally, just don’t reply at all.)
If something can be interpreted multiple ways, give people the benefit of the doubt. If you ask a question, make sure it’s not a loaded one, and try not to make any assumptions. Don’t be afraid to admit that you’re wrong or don’t know something. If you’re so invested in a topic that you can’t discuss it without losing your cool, feel free to skip the thread entirely. Apologize when it’s called for.
TL;DR do your bit to normalize civil discourse. That alone will encourage others to do the same.