Depends entirely on whether you wish to deploy it for Web or whether it is a Stand Alone game.
If it is for web, you could store the data in some Database (MySql) and send the data to Unity using the WWW class - Eg: http://www.unifycommunity.com/wiki/index.php?title=Server_Side_Highscores
If you want to send the data as an XML file, then you can use this forum post:http://forum.unity3d.com/threads/38789-Unity-gt-Php-gt-MySQL-gt-Xml-gt-Unity
For Stand Alone Games, you can write a Save file and use the File reader to access the file.