Apache2 from the Ubuntu repository gets it's default location from /etc/apache2/sites-available.

The default page configuration is the 000-default.conf file in that location.

You can either modify that page or use it as a template and make your own configuration file. If you want to have a page with the location of /opt/lampp/htdocs as it's server route you can do that in this manner:

1) Copy the 00-default.conf file to a new name. For an easy method to remember to purpose, you can call the filename, mywebsite.conf to have the configuration for .

Now edit this new page with these changes:

Change from:

<VirtualHost *:80>
        # The ServerName directive sets the request scheme, hostname and port that
        # the server uses to identify itself. This is used when creating
        # redirection URLs. In the context of virtual hosts, the ServerName
        # specifies what hostname must appear in the request's Host: header to
        # match this virtual host. For the default virtual host (this file) this
        # value is not decisive as it is used as a last resort host regardless.
        # However, you must set it for any further virtual host explicitly.
        #ServerName 

        ServerAdmin webmaster@localhost
        DocumentRoot /var/www/html

        # Available loglevels: trace8, ..., trace1, debug, info, notice, warn,
        # error, crit, alert, emerg.
        # It is also possible to configure the loglevel for particular
        # modules, e.g.
        #LogLevel info ssl:warn

        ErrorLog ${APACHE_LOG_DIR}/error.log
        CustomLog ${APACHE_LOG_DIR}/access.log combined

        # For most configuration files from conf-available/, which are
        # enabled or disabled at a global level, it is possible to
        # include a line for only one particular virtual host. For example the
        # following line enables the CGI configuration for this host only
        # after it has been globally disabled with "a2disconf".
        #Include conf-available/serve-cgi-bin.conf

</VirtualHost>

# vim: syntax=apache ts=4 sw=4 sts=4 sr noet

change to:

<VirtualHost *:80>
        # The ServerName directive sets the request scheme, hostname and port that
        # the server uses to identify itself. This is used when creating
        # redirection URLs. In the context of virtual hosts, the ServerName
        # specifies what hostname must appear in the request's Host: header to
        # match this virtual host. For the default virtual host (this file) this
        # value is not decisive as it is used as a last resort host regardless.
        # However, you must set it for any further virtual host explicitly.
        ServerName 
        ServerAlias mywebsite.com

        ServerAdmin webmaster@localhost
        DocumentRoot /opt/lampp/htdocs

        # We must also allow access to the new root directory; by
        # default only access to /var/www is allowed.
        <Directory /opt/lampp/htdocs>
            Require all granted
        </Directory>

        # Available loglevels: trace8, ..., trace1, debug, info, notice, warn,
        # error, crit, alert, emerg.
        # It is also possible to configure the loglevel for particular
        # modules, e.g.
        #LogLevel info ssl:warn

        ErrorLog ${APACHE_LOG_DIR}/error.log
        CustomLog ${APACHE_LOG_DIR}/access.log combined

        # For most configuration files from conf-available/, which are
        # enabled or disabled at a global level, it is possible to
        # include a line for only one particular virtual host. For example the
        # following line enables the CGI configuration for this host only
        # after it has been globally disabled with "a2disconf".
        #Include conf-available/serve-cgi-bin.conf

</VirtualHost>

# vim: syntax=apache ts=4 sw=4 sts=4 sr noet

After you have created the virtual host for your website, enable this new configuration with:

$ sudo a2ensite mywebsite.conf

The system will then prompt you to restart the server for the changes to take effect, which you can do with:

$ sudo systemctl restart apache2

The two important changes are:

1. ServerName
2. DocumentRoot

Now you can you will be able to access your website by the name given in the ServerName or the ServerAlias directives.

I left the comments in the configuration files example to show the options that can be enabled by removing the "#" desired option. Also the comments are a valuable resource explaining the configuration options.