After the nerves, after the jumbled thoughts, after you don't even remember what was being said for the past 30 minutes, after the interview there's a few things you should remember to do like...
If you can still sort out what was said, what was asked, and how you answered, I would suggest quickly jotting it down if you will be planning to interview at more schools after your first one. It is a great resource to have some questions to review, and if you don't believe you gave your best answer, write down another one you would be proud to give if asked again!
Don't forget to ask for business cards from your interviewer(s)! So many thoughts are shooting through your mind you could easily forget the name of your interviewer(s) after being introduced 30 minutes beforehand. Now you not only have the names, but you have the proper mailing address of those you interviewed with!
Thank you note
THIS IS CRUCIAL! I've been told this makes a great impression for any type of interview including a graduate school interview. Plan ahead and buy a pack of thank you notes from your pharmacy or Walmart for an easy transition. Immediately after the interview, or once you get home, write a brief thank you note that shows your appreciation for them giving their time and appreciation for an opportunity for a spot in the program. Address this and send this out ASAP. Admissions decisions can be made quickly or slowly, either way you want them to recall and remember you for something good.
Call them, email them, write them about your status until after some communication has been given to you first. Admissions committees are so busy from the beginning of the application cycle until the new class begins their first semester. Be respectful of their time.
Most of all, take a deep breath and be proud of yourself! You've made it through one interview successfully. The interview was not as bad as you thought it was and now you're even more confident for the next one. Begin the same preparation for the next scheduled interview!