can't install Moodle on Mac

I've not dodwnloaded or used Moodle before. A downloaded XAMPP (should I have downloaded MAMP? - couldn't see any advice on this), opened it in Applications, clicked XAMMP, was invited to start Apache, MySqL and FTP. But for "Start Apache" I get an error message "XAMPP's Apache cannot start while another webserver is using port 80. Please turn it off and try again". I have no idea what this means / how to "turn it off" etc.  have searched the forum but can't find any help on this; any help would be very welcome.