One of the keys to making a good show is having good sound quality — and that comes from having a good microphone.
What Will Not Work
As handy as they are, an iPhone headset or the mic built into your computer will not suffice. Many listeners will simply turn off the show if it's only phone call-quality sound.
Interested in Getting a Mic?
If you don't have a good microphone yet, luckily a simple Logitech USB headset will sound pretty good. They usually cost around US $30-40, and can be found at any computer or office supply store. Dan Benjamin specifically recommends the Logitech ClearChat Comfort/USB Headset H390.
If you are interested in buying something of a higher quality, look at the Rode Podcaster Dynamic Mic. It's a USB mic, so you can plug it right into your computer without any additional equipment. You can buy it alone, or in a kit with a shock mount and stand. Many people on popular podcasts use the Rode Podcaster. It's also great for doing screencast videos and conference calls.
Already Have a Mic?
There are many other "podcast microphones" out there on the market. Maybe you already have one. Some sound good and some sound awful, so we should do a test well before the show date to make sure we can get a good sound out of yours. Let me know, and we'll setup a time.
Want To Get Deeper Into Equipment?
If you are interested in learning a lot more about podcasting equipment and microphone options, Dan Benjamin has a great guide at: http://hivelogic.com/articles/podcasting-equipment-software-guide-2011. I use a Heil PR 40, and the love it.
If you have any questions about this, let me know!