You need to get the video output signal from the PC to the same type of connection that the LCD uses.
The LCD probably accepts composite video or S-Video input.
You need to convert the PC's VGA signal to one of those.
If the PC doesn't have a composite video output, you'll need a VGA-to-composite adapter, and they tend to be pricey.
And don't bother with one of those $5.00 cables. They only work with certain makes/models of video cards.