Just put the container div around your existing content, for example:
Code:
<div id="container">
<div id="header">
<p>blah blah blah</p>
</div>
<div id="content">
<p>blah blah blah</p>
</div>
<div id="footer">
<p>blah blah blah</p>
</div>
</div> <!-- end container -->



