Need help with soundcard interface

I have made a preamp circuit form a Jaycar kit so I can use my PC as a oscilloscope. I have got to the stage of wiring this thing up. My question relates to the polarity of the plug that connects to the "line in" port on my PC
Does anyone know which way the plug should be wired. It is a standard 3.5mm jack plug. Which part of the plug has the ground wires attached to it.
Also is there a chance I could damage my PC motherboard if I get this wrong?. This is my first post. I am just getting into this hobby with a view to creating systems for regulating power from alternative energy sources such as wind and solar power.

You can't measure DC with your soundcard.

The signal into the soundcard is an AC signal

let me clarify.
This is another idea I have been working on. The input to the soundcard will be a square wave that I will need to observe