Activated iPhone 4 on someone eles itunes, how to take it off that and on mine?

Had trouble activating my phone last night, when i called to activate my iphone 4 and deactivate the 3gs, it deactivated the 3gs and wouldnt let me activate the iphone 4 until putting it into an itunes. I had forgotten when my imac was fixed the guy downgraded it to Tiger. So desperately needing my phone i just plugged it into my brothers macbook and unplugged it after activation. Installed snow leopard on my imac today so now i can use it in my itunes. How do i remove it from my brothers account and onto mine? It only charges when i plug it into my computer. Thanksss

All it does is charge when plugged into my computer. Theres no prompt to remove anything. The iphone uses my brothers login/password. I need to remove that completely and sync it with mine.