http://whotohate.com/nail-mixer/
Nail Mixer
How Do I Make Music Like Trent Reznor From Nine Inch Nails?
Just a Laptop With a Mixer?
You need synth software (such as the Sytrus) or a real synth to start. Then you would need an editing and processing software such as Adobe Audition. You have to pay for all three, and they are expensive, so you would most likely want to be serious about it.
P.s. Nine Inch Nails are the best! 
