The location of the Apaches configuration files depends on how you installed the server. If you installed the official package the files will usually be located in /etc/apache2/
The vhost configuration is located in /etc/apache2/sites-enabled. The file you need to edit when configuring virtual hosts is likely called default or 000-default.
The apache2.conf file is located in the root of the /etc/apache2/ directory.
The mudule configuration files are located in a subdirectory of the /etc/apache2/ directory, but you should not normally have to modify the files manually. Instead try using the terminal commands for installing, enabling and disabling apache mods. I.e.:
sudo apt-get install libapache2-modsecurity
To enable a mod type the following:
sudo a2enmod module_name
And to disable mods:
sudo a2dismod module_name
Remember to restart apache after making changes. See also: Starting, Stopping and Restarting Web-servers in terminal