Open the terminal.
You said you have placed the XAMPP archive on the desktop, so therefore you have to make sure that you have navigated to your desktop folder in the terminal:
Extract the files from the archive using the following command:
sudo tar xvfz xampp-linux-1.7.3a.tar.gz -C /opt
You would probably have to make sure that you have permissions to put files in XAMPP's web page directory:
sudo chmod a+rwx /opt/lampp/htdocs
Starting the server
To start XAMPP, enter the terminal and type the following line:
sudo /opt/lampp/lampp start
Make sure that the server is successfully installed by navigating to http://localhost/. You should see a page that tells you that XAMPP was successfully installed.
To stop the server, just type this:
sudo /opt/lampp/lampp stop
Setting up your web pages
Open the file browser and navigate to the
/opt/lampp/htdocs directory. Place all your web pages in this directory.