/jp/ - Otaku Culture

That's a tricky question, OP. Every image board has it's own little subculture of nonsense that it uses to condense common experiences and communicate with itself when tourists arrive to litter and not contribute. To know what nonsense is currently attached to what level of irony, you have to watch how other people use it for a while, and infer it from context. You may also have to use google and the archive sometimes. In /jp/'s case, any use of greentext is taboo, so >mfw is inappropriate.
When posting your own material, one thing that helps to form a suitable reply is the implied purpose of all image boards, entertainment. The only reason you SHOULD be posting something to begin with is if it is entertaining and on-topic, e.g. funny, hawt, horrifying or stimulating. When reacting to something else, the most universal approach is to make a wise-crack. Another common one is an entertaining image in some way related to the material you're reacting to. It's always helpful to shoop something, even if you suck at it. Shoops are like fiber on an image board. If you absolutely can't think of anything witty or informative to say, or any such image to post, then why are you posting at all? Wait and see how other people react, and maybe you'll think of something, or you'll think of a good reply to their reactions, and start a dialogue of discussion, silly shit, shooping or all three.

TL;DR, lurk moar.