How do you do it? what file do you open to change it?

The short answer is:  "Default.aspx" and "Default.aspx.vb"

The longer answer is:  The HEAD content is dynamically built by DNN and you typically don't want to modify any of the core DNN code.
 

If you want to add meta data tags for page description, use the SETTINGS of your DNN page to add Description and Keywords.  Beyond that, the best way to go when modifying the HEAD is to dynamically inject your code into the HEAD via a third party module. 

Here's one such module:  http://www.dnnstuff.com/Modules/InjectAnything/tabid/309/Default.aspx

Hope that helps-
-mamlin

What I want to do is add a popout navigation menu to my home page but it is dependant on a javascript and a css file...I want to edit the head to include those files.