Will a AT&T micro SIM card work on a t-Mobil iPhone?

Will I most likely have to unlock the iPhone if yes then where

Not without unlocking the phone it won't. There's bound to be someplace local to your un-named location that an unlock the phone for you. T-Mobile can too, if you've been with them long enough they will.

If the phone is unlocked which it probably is. At&t and tmobile both use GSM which means that they use sim card unlike verizon and sprint most of the time (excluding iphone 5 and up). I don't see why it shouldn't because all you should really be doing is taking out the At&t sim card and put in a tmobile sim card.

I will help you.
In order to put your AT&T sim card on your iphone means, Make sure your T-Mobile based iPhone is unlocked and you will need to convert the respective sim size. Because currently you own Micro size sim card. But Iphone accepts Mini size som cards only. If you have locked iphone, try to unlock your iphone using http://www.unlock-zone.com/apple-iphone/rs5wp9/ when i had the same situation, I followed the same way.

Hi, if you are the original owner, then try AT&T directly, for free (first link). Or the the 3rd party service. You need to factory unlock your phone.