You’ll generally add that line of code just below the line referencing the header div in header.php; that is, just below a line similar to this one in the default theme:
<h1><a href="<?php echo get_option('home'); ?>/"><?php bloginfo('name'); ?></a></h1>
(that’s the line in the 2.1+ branch); or this:
<h1><a href="<?php echo get_settings('home'); ?>/"><?php bloginfo('name'); ?></a></h1>
(that’s the line in the 2.0+ branch).
Note that line can vary depending on theme.
In your style.css file, if there’s no .description set up, add one (usually under the #header element is where I put it).
So here’s what I do sometimes:
.description {
color:#FFF;
font-size:90%;
float:right;
margin-right:5%;
text-align:right;
}
Mind, you’ll want to tweak that for however you want it to look, where you want it to display in the header etc.