Hi there!
There's several ways to go about this.
The first thing you need is a microphone. A good one. Doesn't have to be expensive! But you want it to be good audio quality and maybe even more important: solidly built.
I can recommend something like a Blue Snowball. Don't know if you live in the States, but you can find one here:
http://www.bhphotovideo.com/c/produ...LL_ICE_Snowball_USB_Condenser_Microphone.html
It runs via USB, easy as can be! All you need now is software. You can use GarageBand (you've probably seen it in your dock), Skype for calls and SoundFlower (or even the buil-in audio/MIDI setup utility) for audio routing. And that's all free! Awesome!
If you want to get a setup that allows for land-line calls though, you need some serious hardware.
Here's such a setup from that same handy dandy website:
http://www.bhphotovideo.com/c/product/772793-REG/Audio_Technica_THE_TWO_PERSON_Audio.html
As you can see, that's quite the price difference! And it comes with a bit a of learning curve too.
The Snowball is a very cool mic for the money. You can't go wrong with it.
Oh, one more thing, I found this cool article on Mashable with lots more tips:
http://mashable.com/2011/03/25/podcasting-tips/
Good luck!
PS: I don't work for B&H lol.