How to get my Iphone 3g to work in the US?

So heres the situation: Me (German) currently residing in the US. I own an old, unlocked Iphone 3g wich works just fine for making calls to Germany but i'm unable to talk/text with people living in the US.
After doing some research i believe to have narrowed it down to me needing an US sim-card for my unlocked phone.
My main problem is, i understand next to nothing about mobile phones, networks and sim-cards.
Exactly what do i need to do how in order to be able to phone with people from the US?
In case this information helps: My phone is showing At&T as it's current network.

You need to either contact Your carrier in Germany cuz they would have to give you access which is probably expensive. If you're living in the US you need to contact AT&T here in the US and they'll be able to set you up with a plan.