Sending data from PC to micro-controller
The easiest way is to use a serial port (or a USB to serial adapter) and use the UART module on the micro-controller. You should send high level commands (step direction, speed, number of steps, etc) for the PIC micro-controller to interpret, the PIC generates the waveforms for your h-bridges to control the motor(s). This is different than projects which use the parallel port, as the parallel port is typically used to generate the waveforms directly. 

A good source for stepper motor control is to look at the hobbyist 3D printer and CAM (Computer Aided Manufacturing) communities (laser cutters, mills, etc). Here is a common h-bridge design used for driving stepper motors: Pololu Stepper Driver. Here is the electronics used to interface several of the drivers with an Arduino (which is fairly straightforward to adapt to a PIC circuit): Arduino Driver Board.