Recording an interview is easier than ever with a smartphone or laptop. Recommended apps include:
External microphones are always recommended, but they may not be necessary with a good app and the good positioning of your device. Be mindful that they can be cheap or expensive. If your interview is for a class project, and not to be saved once the project is completed, you can probably forego the external microphone - just be sure to test your app beforehand, in the space set for the interview.