You need a good mic and a decent mic preamp and a decent USB Audio Interface. This comes both separately and in combinations, including USB microphones that include all in one package.
In podcasts a good sound is essential, and you get that by being fairly close to the microphone, so if you are several podcasters, they should really have one mic each, and then you also need a mixer. In theory you can have several USB mix and mix them in software, but although I never did that and it might work well there is something in my stomach that get slightly worried when thinking about that... :)
If you need only one mic, and you are sure you are only using it for podcasting, a USB mic of some sort would be a good choice. If you want more flexibility get a separate mic and a USB Audio Interface that includes a mic preamp.
To a large extent its a question of budget.
You most likely want either a nice dynamic mic or a large diaphragm condenser, these work best for vocals. I completely agree with Kyle that an SM58 (or SM57, but then you might want a pop filter as well) is a good starter, very versatile and not expensive. You can buy it packages with an XLR to USB adapter for what seems like a reasonable price (