Can I put AT&T Sim card into iphone 4s?

So I have a windows phone from at&t that I got a year ago, and I don't get the upgrade for another year. However the phone sucks and the screen broke and it isn't working properly so I was going to buy an iPhone 4s online somewhere, either refurbished or used or what not. I already have a data plan with this phone, so when I get it can I just put the sim card in the iPhone and it will work fine, or do I need to bring it to the At&T store or call them or something?

I think the Iphones take a smaller sim card than a regular phone. Check with them. You can google what size sim card the Iphone takes too.