Seems to me you understand the software component to your problem. Your output already will be 0-5 volts, all you need to do is the math. You will need to know your desired duty cycle as well. I would load a timer and use a 50/50 duty cycle, meaning same amount of time on as off. You have 1,000,000 machine cycles to start with (12mhz/12=1,000,000) because it takes 12 clock cycles per machine cycle.

Hope this helps.

James