Lemmy works on markdown, so if you want to do anything, just look up “how to do x in markdown.” For example:

itsfoss.com/markdown-images/

If you want to learn more generally, there are many guides, e.g.:

www.markdownguide.org/getting-started/

And if you want to play around without other people seeing any mistakes you make, there are multiple web interfaces as well as local editors that will display your works in real time, such as:

markdownlivepreview.com

Hope this helps!