The most important thing to know is that the forum software supports the Markdown markup language for formatting your posts. There are a lot of excellent references for Markdown, but this seems like a decent one:
Markdown is actually a partial superset of HTML, meaning that, in addition to the primary Markdown markup syntax, you also have access to the fundamental HTML syntax. Usually not necessary, but every so often there is something missing from Markup that I fall back on HTML for. For example, the Discourse software has an annoying handling of the IDs of the automatically generated anchors for Markdown headings, so I use HTML <a> tags to supplement my Markup with custom anchors.
To make matters more confusing, the forum software also recognizes some flavor of the BBCode markup language. I don't bother with that much because Markdown is much more generally useful (BBCode is mostly only used on forums, but Markdown is used everywhere), but there are one or two things BBCode does that Markdown and HTML can't. The notable one is the quotes. This is how a Markdown quote looks:
> Is there a guide to using the forum software?
renders as:
Is there a guide to using the forum software?
That's OK, but BBCode supports the additional metadata on the quote that identifies its source:
[quote="gggghhhh, post:14, topic:921586"]
Is there a guide to using the forum software?
[/quote]
renders as:
Then there are some other additional markup capabilities such as emoji that are not part of any of those languages.