I have an at&t 3G Sim card from a blackjack/SAMSUNG magnet phone and it wont fit into my blackberry 8800?

should i buy another sim card for the phone or ?

Are you sure you’re putting it in right? AT&T sim cards are universal. I would take it to the store and have them see if they can fit it in there. It might be because the 8800 does not support the 3G network.