Refer to the documentation for more details: http://codex.wordpress.org/Installing_WordPress_in_Your_Language
Skip the first two steps and the backend won’t be translated.
1) Download the Italian version of WordPress.
2) Upload the files located in /wp-content/languages/ to your live site. These files are for translating the Backend.
3) Define the language in wp-config.php as define ('WPLANG', 'it_IT');. Before uploading the new version of wp-config.php, make a backup of the version in the live site.
4) The frontend translation is done by your theme and it needs the file: /wp-content/themes/your-theme/language/domainpath-it_IT.mo.
5) Much probably this file doesn’t exist, and you have to translate it yourself. The same goes for any plugin that renders content that needs translation. Normally the translation files are inside a folder called languages, but it could be lang or similar.
6) The easiest way to translate theme/plugins is to use this plugin: Codestyling Localization. Or you can use the software PoEdit, more info in this Answer.