How do I turn a Fitec FS90R servo motor 90 degrees clockwise and anti-clockwise using a PIC18F4550

You need some measurement device to read the angle, if you want to stick with that servo. With the angle you can implement a closed-loop transfer function to approach the angle you want.

Servos as the FS90R are "continuously rotation servos" and the control signals sets the speed (exactly: the driving current, giving the torque) and direction. For example, see this fine explanation.

If you want to control the angle directly by the control signal, you need "the other" type of servo, described earlier in the linked Wikipedia page.