You're PIC will generate the required PWM pulses to your motor driver. Read the PWM section of the PIC data sheet and figure out the duty cycle you want to achieve. In your driver data sheet, you'll find how to interface the PIC with the L298. I suspect you might need to isolate your PIC from your motor driver because of a much higher current flow in the driver.
Let's try a simple test. You dont have to think about PWM yet. Just assert 5V to the enable pin and test if your motor driver can work. This is the simple way to use L298N.
The PWM signal driven out by your PIC to the motor driver (enable) pin. PIC16f877 have a built-in pwm. You need to read about CCP module, which letter 'P' belongs to PWM. There are a lot of thread discuss about PWM issue..You can search for it.