You can do formatting with basic HTML tags, like <b> and <i>. In HTML, a tag indicates where the formatting should start (like <i>) and then another with a / in the front indicates where you want the formatting to end (like </i>).
So for example if you enter this:
<i>here's something in italics</i> and this is in <b>bold</b>
it would look like this on the web:
here's something in italics and this is in bold
There's a good summary of these basic HTML tags, plus a few other useful ones like bullet points, at
http://www.pageout.net/pageout/help/.