Can i put my sim card in an iphone 3G?

HI yes you can put your AT&T sim in iphone 3g only if it is unlocked one .Else if the iphone 3g is locked to some other network provider then until you unlock it you can't use your old sim in it .You can easily unlock your iphone 3g with the help of sites like MobileUnlockSolutions.com here they are rendering remote unlocking service to unlock all model iphone from network lock ....After unlocking your iphone 3g you can freely use it with any GSM sim ...