In order to set everything up, you first have to go to the "Settings" app , then go to "iPod." From there, type in your Apple ID email and password.
Also, make sure Home Sharing is enabled on iTunes (on your computer). You can find this in preferences. You're also going to have to type in your Apple ID and password there, too --just make sure you use the same Apple ID that you used on the Settings app on your iPhone (if you have more than one account).
Once your computer and iPhone have the Apple IDs setup, you can then go to the "iPod" app on your iPhone. Tap the "More" tab on the bottom. Go down to "Sharing," then select your computer (e.g. Michael's MacBook Pro).
That's it. Hope everything works out for you.