can i control a dc motor circuit using ethernet cable? a usb port? a vga port?
Has the laptop got any other ports?
You'd find it easier to use a parallel port (printer port) or a serial port.
The VGA port is out of the question unless you want to totally rewrite the firmware of the laptop, the Ethernet has got almost the same limitations and USB have only got 2 lines that is controlable (and controlling them for logic needs low level code (i.e. Ring 0 code), so is not for those having to ask.
Even with a parallel port you will need buffers, but a single transistor and a couple of resistors will do for each line.