There are a couple of ways to do this, just pick one. But the problem is that you have left and right floated content and the footer needs to clear those. You would add a clearing division or a clearing break to let the footer drop down below that content.
In your footer.php file, before the footer division, put this: <br clear="all" />
Or in your stylesheet, create a division like this:
And in your footer.php file, above the footer division, put this: <div class="clear"></div>
Finally, you can just add this to your footer code in your stylesheet:
[whatever else you have there]