To install iTunes, you need to open your web browser, and navigate to the iTunes website, http://www.apple.com/itunes/download. It should automatically identify your OS and provide you a link to download and install iTunes.
As far as the backup, you can take a look at this support document about backups. It will identify where the backup is located. I'm not sure there is a way to change the default setting for the backup, as I've never attempted to do that. One support document helps you determine iCloud or iTunes for backup, and then where it is located at.
About backups in iCloud and iTunes - Apple Support
Find and manage your iTunes backups - Apple Support