Applies to: Microsoft Teams.
Desktop
There are countless formatting options for messages.
To access your formatting options, press Format below the box where you state your message. In this enlarged view, pick your desired text to format, then select an option like B, I, or U to bold, italicise, or underline the text.

There are even options for highlighting, font size, lists, and more. See More options for extra formatting options, and the buttons under the box for attaching a file or adding fun stuff such as emoji.
Note: Teams underlines spelling errors in red as you type. Right-click the error to see a list of suggested corrections. Currently, you can’t run a separate spelling and grammar check in Teams (unless you’re working in your Wiki tab).