If you use Post Reply rather than Quick Reply, you'll get formatting options, including Bold, italics and underline. That way, you can see the html codes (and write them down for future reference, if you'd rather not use Post Reply).
If you click on any of those options, like underline, for example, you'll get a window in which to type the phrase. After you type the phrase and click OK, the text will be surrounded by the html code. U and /U (square brackets around each) gives you underline. B and /B is for bold, and I and /I is for italics. Again, square brackets around each. (I can't actually write it out for you as it will be translated into code).
Try it and see what happens.