Nokia Lumia 800 - How do I manage and transfer music from my computer to my Nokia Lumia phone?

To manage and transfer your music collection from your computer to your phone you need to download the Zune PC Software or the Windows Phone App (Mac OS) application. 

The Zune application is a program for your PC that lets you sync music, videos, and pictures between your computer and your phone. It also lets you install the latest software updates for your phone.If you have a Mac, you can use Windows Phone App to sync multimedia files between your phone and your computer.

For more information about installing and using Zune or Windows Phone App, visit the Microsoft support pages at:http://www.microsoft.com/windowsphone/en-us/howto/wp7/start/install-the-zune-software.aspx