For hardware -- you can head to Radio Shack and get a four-input mixer on the cheap. This gives you four inputs but one output. I recommend using a decent cardioid mic with a stand. The mixer and mic and cables will set you back roughly $80. For the input from your iPhone, grab a cheap cable to run from your Audio Out jack into the mixer.
The nice thing about the mixer is that you can set the levels using hardware instead of trying to manage multiple, competing input sources directly on the computer. All inputs into my podcast are pushed through the mixer, with the computer/Audacity having to deal with a single input. This makes it easier to export the podcast to MP3, since you don't need to spend as much time working through a file with several different clips embedded in it.
For Skype, there's a plug-in that lets you record conversations. Use that, and just incorporate the WAV file into Audacity. Head to Skype's in-app menu for plugins to download any which one you prefer.