As others have said, you need a mixer. I sometimes mix signals inside the Zoom H4, because it has a mixer in it and plugs into my USB port. You don't have to have a USB mixer, since I used to use an old analog mixer that plugged into my microphone input on the computer - very old school.
Guitar Center has a good deal on the Tascam D-004 for the next few days ($129 with a $20 rebate, plus the sale). It comes with a couple of pretty good mics, plus a 4 channel mixer.
I usually just use Audacity as a mixer, but when I do I record the tracks separately and then mix in my computer. Its a little harder if you don't have a good mixer, because you can't necessarily monitor the tracks that you just recorded.