By default the document root folder for apache2 in Ubuntu is /var/www. This is where you can store your site documents.
In order to change the default site location to a different one, /opt/mysite use the following method. A detailed steps to install LAMP on ubuntu is given here.
To do this, we must create a new site and then enable it in Apache2.
To create a new site:
Copy the default website as a starting point.
sudo cp /etc/apache2/sites-available/default /etc/apache2/sites-available/mysite
Edit the new configuration file in a text editor “sudo nano” on the command line or “gksudo gedit”, for example:
gksudo gedit /etc/apache2/sites-available/mysite
Change the DocumentRoot to point to the new location. For example, /opt/mysite (make sure there is no space in your new folder name; /opt/my\ site/ will not work)
Change DocumentRoot /var/www to DocumentRoot /opt/mysite
Change the Directory directive, replace <Directory /var/www/> to <Directory /opt/mysite/>
You can also set separate logs for each site. To do this, change the ErrorLog and CustomLog directives. This is optional, but handy if you have many sites
Save the file
Now, we must deactivate the old site, and activate our new one. Ubuntu provides two small utilities that take care of this: a2ensite (apache2enable site) and a2dissite (apache2disable site).
sudo a2dissite default && sudo a2ensite mysite
Finally, we restart Apache2:
sudo service apache2 restart