Wherever you bought the phone, you should be able to take in your old phone and they can move transfer your contacts from the old sim card to the iPhone sim card.
The only way that I have seen to do it at home is with a Jailbroken iPhone, there are programs which will sync your contacts from the old sim to the phone. But, I would suggest taking it to the store. It's much easier.
If you'd like to jailbreak, go to the Jailbroken forums here at TiPb and read up on the guide they have.