Above the message field there are three icons for bold B, italic i and underline u. To start writing in bold, for example, click on B - this will add a [b] symbol to the text, which means that any text that follows it will be displayed in bold. To stop writing in bold, click on the B symbol again - this will add a [/b] symbol which signifies that the bold type has finished.
Click on 'preview' to see how your message will be displayed - the text should be displayed in bold.