In your Apache configuration file, find a line with 'DocumentRoot' and modify accordingly.
Regarding your comment, then you're looking for this:
Though for the virtual-hosts request,
DocumentRoot /var/www/sitename #STILL DOCUMENTROOT!!!
Options Indexes FollowSymLinks MultiViews
allow from all
# Possible values include: debug, info, notice, warn, error, crit,
# alert, emerg.
CustomLog /var/log/apache2/access_sitename.log combined
Though note still, it uses the
DocumentRoot directive. You can have as many sites as you want. Modify the paths to match the
httpdocs/public directory, and I assume that inside /public/ there will be directories for other virtual hosts?
If this isn't what you wanted, please re-phrase your question.