You can use special formatting in your instant messages to make text bold, italic, strikethrough and monospace.
Put *asterisks* around the words you want to make bold.
Put _underscores_ around the words you want to emphasize.
Bookend your text with ~tildes~ to put a line through it. You might use this to convey sarcasm or show what's changed, for example.
For that plain text look, wrap {code} around words like this:
You can use two @ symbols or two ! and a space ("@@ " / "!! ") at the beginning of a message to prevent special formatting.