When you're replying to a message (like right now, click on the reply button at the bottom of this message), you'll notice a few links to the left of the message box where you type your message. One of them is for HMTL (which is what Rei suggested), another is for UBB codes, which tend to be a little simpler than HTML, plus they provide a couple different quoting styles and miscellaneous other things.
--Percy