Unlocking iphone 3gs, sim card sequence

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hello. my friend gave me his iphone 3gs from att. he called them and had it unlocked.
i have t-mobile as my carrier.
My question: when i go to hook up the iphone to my computer to sync up with itunes,
do i start with my friends old sim card in the phone or do i need to put in my t-mobile sim card in the phone before i start?
thanks
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