When you get to Tiffany's apartment building, a quick check of the mailboxes will tell you that the door you see at the right is Tiffany's apartment. It's locked and she doesn't answer the buzzer, so it's Joey time.
Unfortunately, Tiffany is both hostile and suspicious. The only thing you'll get the first time you visit is a confirmation that she is indeed Tiffany Walters.
She's never going to really open up to Joey, so you need to learn most of her story from other sources.