How to us phone in Europe with my pre-paid plan

First you should have a world phone. If you have an inexpensive AT&T Android phone it does not have enough connectivity bands to operate in most other countries.

Second, prepaid Roaming In is limited with a $35 add-on to the country's listed here

https://www.att.com/prepaid/static-pages/intl-travel-add-on/

If you are in a country that is not listed you will have no service at all other than Wi-Fi call and text. 

You should be able to log into your account while in covered countries only. Your account is only charged if you choose to use the add-on. I would advise you put sufficient funds in your balance before leaving. Those funds can be used for regular service when you return to the USA if you did not choose to purchase $35 add-ons

Thanks.