I suggest you browse the forum a bit, get familiar with it's subsections. In doing so you may stumble upon some threads which you'd like to contribute or share your thoughts.
Take note of some useful sticky threads in Translations and Learning Japanese subsections.
If there is a specific topic you'd like to discuss, and you can't find similar threads via the forum's search function - feel free to start a new thread in a relevant subsection.
Abstain from necroposting though, since it is frown upon around here. If a thread looks dead or concluded long time ago - it may be wise not to post further replies to it.
Aside from that just use common sense. The forum is quite liberal and even outright ridiculous opinions and controversial topics are allowed to be discussed, as long as they are held in accordance with the forum rules.