1. Choose the amount of money you are willing to spend - I am choosing all USB microphones for simplicity sake
Step2
Blue Snowflake
If under $60 and this is the cheapest I would go, get the Blue Snowflake USB Microphone from BlueMic
Step3
Blue Snowball
If you can go to about $100 - the move up to the Blue Snowball also from BlueMic
Step4
Audio-Technica AT2020 USB Mic
Moving up a step to the $150 range is the Audio-Technica AT2020 USB Condenser Mic
Step5
R0DE Podcaster
For going very nice and high-end on USB mics, is the R0DE Podcaster USB Mic. This is really the best you need to go unless you start going for outboard gear (mixing boards, compressors, pre-amps and such)
Tips & Warnings
The microphone is the single most important thing to sound quality.
Get the best microphone you can afford for your show.
All the mics mentioned above are available many places - but I do trust musicians friend