Links to communities should use this format [email protected] so that clients will open them properly. https://feddit.org/c/main this format will not work properly. You can not do the same for links to posts or comments tho so you have to hard link those sadly.
To embed an externally hosted image like this you can use this format 
You can ofcourse just upload the image to your instances media storage, but some instances have that disabled or have file size limits.
To embed a hyperlink into text you just Ctrl+C the URL, select the text that should contain the hyperlink and then Ctrl+V which will add the link to that selected text.
If you want to force a new line while writing text, you have to put two spaces at the end of the line.
like this
Otherwise you either need to put a full empty line in between or it will just be continuous flowing text.