When it comes to starting a microphone you don't need to spend hundreds of dollars on a single microphone. Some of the best “starter microphones” for podcasting are
All of the above microphones with via USB or you can plugin them into a mixer.
You need a mixer when:
If you don't have one of the above scenarios you don't need a mixer (but they are nice to have around). You can add bass and treble to your voice and other items.
Speaking of the sound of your own voice, we will be talking about why most people hate the sound of their own voice in a future episode.