There is a very simple guide for creating new AT&T net email login account in a few steps, follow them one by one:
Go to currently.att.yahoo.com and click on my AT&T from the menu bar.
Now, click on my AT&T drop-down arrow button on the top right corner of the screen.
Click on the sign-in button from the list.
The sign-in interface has appeared in front of you and you have to choose don’t have a user id, create one now.
Just follow the instructions and create your new account.