Hi to the Ohio Valley.
I think to give you a decent advice of what to do, you have to state what you want to do with your European SIM: calls, texts, data? Any idea how much and where to?
1st you should find find out (as Inquisitor has said) if your device has a SIM-lock or not. This is kind of tricky in the US. If your phone has GSM on 850 Mhz or 1900 Mhz, you can put in any AT&T or T-Mobile US SIM card and check if it connects to an network. But with cheap phones from Europe, they often cover only the European bands of 900 and 1800 Mhz (check the specs).
Assuming that the phone is SIM-locked, you can only put in the SIM cards of the provider (or with a NET-lock) of the network operator. If you can't get it unlocked, check if the given German provider has an EU package offer which might be the better deal, if you use it only lightly.
If it isn't locked or you find someone to unlock it (ask around in the "independent cell phone stores" run by immigrants in your country), you have the choice of adding new SIMs.
But have in mind that the 2 suggestions of Toggle Mobile and Piranha are only good for voice and text. It's a total different story if you want to use (a lot of) data and can channel your calls through VoIP (like Skype). For this, you should look at
prepaidwithdata.wikia.com