By default the Document Root directory will probably be owned by root. Therefore you will only be able to modify files there if logged in as root.
There are a couple of ways around this. You could create a group called, for example, web and add yourself to it. Then as root do the following :
chgrp -R web www
chmod -R g+rw www
A simpler solution for testing might be to use user directories. Most Apache installations are set up by default to give access to a directory called "public_html" directly under any user's home directory. To access such a site the URL to use is :