A podcast can be just your voice. Many Podcasters default to a simple presentation either because they choose an easy method or know the information they are providing does not need enhancement. But, you may want to use a pre-recorded show intro, with various inserted pieces of audio, possibly commercials, etc.
Many Podcasters use Audacity, a free and open source multi-track recorder which will at minimum allow you to record, edit and mix your voice with music or sound effects. Audacity comes in both Windows and Mac versions and you can download it from the Audacity website.
Another useful program for audio editing is Kristal Audio Engine, a multi-track recorder that is also free. Visit the publisher's site for more information.
Finally, let me suggest you look at Wavosaur, an audio editor which is also free. More information and download is available at wavosaur.com.
Although mixing can be done with any multi-track recorder like Audacity, there are some program designed to assist in mixing. One program is WildVoice which allows you to record your voice and mix in music and sound effects while you're talking. You can then edit and "mix" your audio into one final sound file. WildVoice is free. See more on WildVoice here.
PodProducer is a handy little application which is very easy to use and surprisingly robust. It employs various features which allow for easy recording and audio mixing along with some nice enhancements which can make a Podcast sound pretty professional. See more about PodProducer.