hi dear try this and see whether you can do this on your own. your project consist of two part one controlling the stepper motor and other communicating with PC through usb.
regarding controlling of stepper motor there are lot of circuits available on the net.
and for second part you can use either usb to serial converter to your micro controller or a micro controller with usb capability. the later one will be difficult in programming i suggest you to use the first one.
regards
kj