Adding widget area to header

Hello, so I’ve been tooling around with some other threads and how-to’s and have had zero luck with this specific topic. I’m looking to add a widget area to my header so that I can place an ad in the header.

URL: https://thatwasjunk.com
Theme: Twenty Twelve

Can anyone give me a thorough how-to on adding this widget? I used a tutorial from another site and it messed up my functions.php file for a couple hours.

Thanks.