Acoustic foam, even tho it's name suggest it, is not the best acoustic threatment to use.
Foams will affect high frequencies and not mids and lows and this mostly give unbalanced results.
The best solutions are dense fiberglass pannels or Roxul made (at least 3-4 inches thick)

But, doing podcast isn't critical as recording or mixing an album. Within your small room, you will mostly get flutter echo and un-attractive roomy reverb.
Getting rid of those can be done with any surface absorbtive material like moving blankets or others.

One other thing that can help is to use a dynamic mic instead of condenser. in an weird sounding room, dynamics do a better job.

mactreouser, post: 448297, member: 47968 wrote: What type of acoustic foam should I apply on the wall (Rear and Back) ?

You should also do something for the sides. The first reflections are the ones that will help the most.

To inspire you with designing your space, listen to that serie of videos from rode. (althought I don't agree with some of the information, for small podcast, it's gonna be a very good start)

pcrecord, post: 448303, member: 46460 wrote: You should also do something for the sides. The first reflections are the ones that will help the most.

...as well as the ceiling. ;)