You can do one of two things:
1) Change the permissions of the /var/www directory (by typing
into the terminal)
sudo chmod a+w /var/www
2) Point the server to a different directory. This you can do by editing the apache2 config file:
sudo gedit /etc/apache2/sites-available/default
to your desired folder name.
for example, if you create a folder in your home directory called 'www', then you would change it to