Can I use an AT&T iphone 4s using a Tmobile sim card?

I dropped my phone and broke it, so my friend is giving me her old iphone 4s. Her contract already ended with AT&T, and I heard that once the contract is over, the phone is automatically unlocked and able to be used with any carrier. So my question is whether the phone will work properly with my T mobile sim card and if the phone is unlocked automatically or do I have to jailbreak it myself or bring it to an AT&T location to have it unlocked?

You answered you question all by yourself, Amanda:
Yes. You have to take it into an ATT&T store to have it unlocked. And, I'd also suggest that
you terminate your T-Mobile account as soon as possible, if you have not done so already.
(Then start an ATT&T account of your own, as easily as would be possible for you.)

No, the phone is not automatically unlocked. You would have to request that it is. Your friend should do it though & then you get it afterwards.

It will not work with a T-Mobile SIM unless it is locked. A jailbreak is not absolutely necessary for an unlock, not at all if it is done through the carrier.