Your best bet is to have a mixer to handle your inputs and routings to speakers and outputs, and feed its main output to an audio-to-USB interface - Roland make a range of these (they took over Edirol and I've used a couple of their interfaces for years, but those specific models are no longer availabl).
There are also mixers from Alesis and Behringer which have a direct USB connection, which would save you one box.
This page lists some interfaces:
If you want to ask more about the the best place is the GarageBand forum - there are a number of people there who can advise on current equiptment.
As to podcasting itself, you may find it helpful to read my page here: