A few tips I've found that make a big difference for our guests:
If you will be on a PC/Mac, here's a low cost mic that works very well.
If you will be using your phone or an iPad, you can try to get a USB adapter like this to use the mic above, or get one that is specially made for mobile recording.
If you will be on video, I recommend a web cam light and the center cam so you are looking at the camera during your interview.
Use EveryTimeZone.com and reconfirm with the host.