At the time being Flarum doesn't come with a visual editor. But Flarum supports Markdown nevertheless.
Have a look here: https://github.com/adam-p/markdown-here/wiki/Markdown-Cheatsheet
Not everything is supported by Flarum yet (e.g. tables are not), but most markup is.
To format text bold, just put it into double asterisks front and end:
This is **bold** text.
... becomes:
This is bold text.
You will see the results in the preview while writing your posts.
