How to create a 2nd itunes account on the same computer?

I have just got an iphone and have created an itunes account for myself. My boyfriend already has an account for his iphone which has all his music and apps and everything on. When I log in to my account on our laptop all of his things come up in my account. I have now created a 'guest' account on our computer but it wont allow me to burn my CDs on to it in itunes… Any ideas at all?
Thanks

Create a non-Admin User Account for your boyfriend.

Then, in your iTunes Account, delete his iPhone, and Activate yours. Then, change your Admin Password so your BFF can only make changes to his own User Account, and can never log in to your Admin Account again.

He can log-in to his new User Account, and open his iTunes, and Activate his iPhone without messing with any of your settings.