For your recording purposes, you want a microphone with a cardioid polar pattern. That just means the microphone is more sensitive in the front (where your mouth is) than the sides or back. This type of mike will help to accentuate the positive qualities of your voice while cutting out room echo and other noise.
There are a lot of microphones. My suggestion is to talk to other Podcasters or read some reviews at Amazon.com on the microphone you are interested in.
As a starting point, here are some USB microphones to consider:
Blue Microphones Yeti USB Microphone (Compare Prices)
Audio Technica AT2020USB Condenser USB Microphone (Compare Prices)
Shure PG42USB USB Condenser Microphone (Compare Prices)
Shure PG27USB USB Condenser Microphone (Compare Prices)
Blue Microphone's Snowball USB Microphone (Compare Prices)


