Well,
First you'll need to remove margins, so it actually goes all the way across. Do this by going into Main CSS in your Style Manager.
At Body (The First One), change the Extra CSS Attributes box so
margin: 5px 5px 10px 5px;
reads:
margin: 0px 0px 0px 0px;
This removes the small spaces around the edge of the page.
Next, save, and go into your header template.
Now just create a table with a width of 100%, e.g.
<table cellpadding="0" cellspacing="0" width="100%">
<tr>
<td align="center">Header</td>
</tr>
</table>
<br />
The rest of the template should just be normal. There might be a slight change due to the margins being revoked, but this makes it actually go all the way across.