How can I get copy Text messages

You can’t, at least not from ATT. As noted above, ATT does not save the actual texts. All you can get is the text log which contains the date, time and number only, not the text content. Text content is only on the phone.

AT&T doesn't keep the actual text messages (it's a privacy issue),  only when they were sent or received. You need to look at the phone.

@Melissakay1 

Text messages, just like phone calls, are private to the user. As the account owner, it is your right to see the number that they called or texted with. The content of those calls or texts is not recorded as would violate privacy.

You would need the actual phone to look at it and read the messages. Though I hope it is for a very good reason that you violate someone's private conversation.