Spend some time browsing the forum, familiarize yourself with the tag system, and do a few searches for keywords related to your topic: it could be someone has already started a discussion about it!
When you're sure you're ready to start a discussion, please keep the following points in mind:

Give it a good title! You'll get the best results if your title makes it clear what you want to talk about.
Choose the right tag(s). This will increase the likelihood your post will be read and answered promptly.
Don’t post repeatedly about the same topic, as doing so will tend to have the opposite effect.
If not using a tag set aside for multilingual use, post in English only. We can't help you if we don't understand your posts.
Remember, you don't need to sign your posts. We've got your profile to let us know who you are.

If you reply to a discussion

You're taking the time to participate in a discussion, in the hope that others will read your ideas and take them into consideration. So why not make the effort to make your reply worth reading?

Don't reply to a title. Take some time to read the original post (OP), and at least scan the rest of the conversation first.
Ask yourself if your reply adds to the discussion. If it doesn't, give it some more thought before posting.
Avoid making one-word posts just to agree with someone; you can use the "Like" button for that.
Avoid making multiple posts in a row when one would suffice. This is a forum, not a chat room.
If your reply is likely to divert the course of the discussion, consider starting a new discussion instead.
If you just want to post a bit of nonsense as a test, please do it in the Test Posting tag instead.
Make sure your replies provide constructive feedback and support to allow for an inclusive community.

No one's going to complain about the occasional joke or smart remark. We like to keep the mood light! But to keep things productive, as well, we ask that you try to avoid derailing a discussion altogether.

Thanks to Flarum for these guidelines