I checked the back of my computer, and I found three slots for plugs (all beside eachother). One is black, one is red, and one is blue. The black one appears to be the computer's output, because it leads to the speakers. I have no clue what the red and blue ones are for, though, 'cause there's nothing plugged into them and there are no icons.
I don't know what kind of cable exactly you're talking about though.
It's a basic stereo cable. There's a red plug and a white plug on both ends.

I was thinking of maybe plugging the red plug into the red slot and the white plug into the blue slot. Or, I also have a stereo "Y-adapter" which can turn my red/white plugs into one single plug. Maybe I could plug that into either the red or blue slot.
The problem is that I have no way of telling whether or not the computer is actually receiving the input. What program(s) would I need to record or test the connection?