Garageband is an awesome tool to use for your podcasts.  It is free, easy to use, yet has some extra features that you can use if you want to become an audio expert.
These two videos should help you out.

For free music (Bumpers) head to BenSound.com

Helpful Links

• YouTube Sound Effects for you to download free (without using KeepVid)
• KeepVid.com  Makes downloading YouTube files easy
• BenSound.com  For free, awesome bumper music.

Tutorials

Getting Started

https://youtu.be/tbDlSmpYItg

Download Sounds from YouTube thanks to KeepVid.com

https://youtu.be/xBquFKzG_ps

Adding Bumpers

Export when finished

https://youtu.be/D7XXn2AWRI8