Take a deep breath; you did it! Once the interview is over, it may be helpful to jot down any notes that you might want to look at later, from pieces of information to the general feeling you got from the interview.
Some people may send thank you notes or emails to the programs. I did not do this, but if that’s something that feels right, go ahead! You can generally address it to the program in general if sending a note via snail mail, or to whoever set up the interview if sending the email.