Building a intermediate op-amp
I don't think you want to build an op-amp.  An op-amp is an electronic component that you usually buy already-made and build other things with.  A typical op-amp IC looks like this:



I think what you want is to build a headphone/line amplifier with an op-amp.  There's a popular guide here:  http://tangentsoft.net/audio/cmoy-tutorial/