click the "edit HTML" checkbox thing on your xanga and then put in this:
for bolding:
CODE
<!-- begin code provided by createblog.com -->
<b>your text here</b>
<!-- end code provided by createblog.com -->
for italisizing:
CODE
<!-- begin code provided by createblog.com -->
<i>your text here</i>
<!-- end code provided by createblog.com -->
for underlining:
CODE
<!-- begin code provided by createblog.com -->
<u>your text here</u>
<!-- end code provided by createblog.com -->
i'm pretty sure it's the same everywhere, it's just HTML, u'll get the hang of it