To format the text within your posts, use the buttons which appear under the subject bar.
If things don't end up as expected, it could be because the 'code' in your post has mixed up.
This is the correct syntax for formats:
[formatcode] text here [/formatcode]
Where formatcode can be:
b for bold
u for underline
i for italics
quote for quote
It's Starting the formating with the [ ]
and Ending the formating with the [/ ]
Hope that helps. Also, here are some images to help illustrate. The first shows the text that goes in the posting box, the second shows the resultant text.