A ad/da converter is a hardware device which needs to be purchased. The most simple ones do have 2 channel stereo.
There are many different brands with many different features and qualities/prices out there, the one I recommended above is this one: http://www.m-audio.com/products/en_us/MTrack.html
As I said, just one example. There are too many.
I am not an expert either, just had the same problem as Natty Matti once.
I went with this combination Audacity /M-Track and I am fully satisfied with it.
I believe this topic is dealt with already on the board (or I read about it elsewhere).