Micro sim /iPhone query ?

[BeaLola]

I have no idea about phones, it etc so need to know what Inshould be asking for.

My brother has upgraded his iPhone and given me his iPhone4s if I want it to replace my old mobile. I'm not on any contract.

I thought this would be good as I do have an iPad and thought it would be worthwhile using this phone. Thing is do I need to go and buy a new micro SIM card which I understand iPhone 4s use with whoever I decide to buy my mobile service with eg BT , Vodafone etc ? Or will his old one just work with whichever provider
I chose?

Thanks if you got this far, not even sure I have asked the right questions !

[Gemauve]

If you're on PAYG, just go and pay a pound (or whatever it is) and get a new MicroSIM.