SMPS Buck converter with PWM circuit
Without researching your circuit i want to warmly recommend something. Prepare three stages on your feedback, and have their sum on another opamp. One stage is buffer, another one is integrator and the third one is derivator. Each should have a nice convenient potentiometer to set the gain. Then you will be able to tune your system for best performance.

Edit

Most importantly, simulate before you go to solder anything!!!

Edit

Based on assumption that your components are appropriate for voltage and current, and that you know the basics, I will explain what you should do to close the loop.

I will add a little bit more info. First of all, your PWM controller must have a saw-tooth waveform of constant frequency and "command signal" which is basically voltage, that you compare to this saw-tooth. If the command is smaller than the saw-tooth, your switch is on, otherwise- off. Make sure it works for a constant command, this is an open-loop PWM controller.

Now you have to close the loop. Closing the loop basically means making your error zero. So you have first of all to measure it by subtracting your output voltage from a reference voltage. Reference is generated by a zener diode with resistor. Or you can take an IC that will do it inside more accurately. The output voltage is usually higher than your reference, so use resistor divider, so error is 0 when the divided voltage equals to your reference.

Now when you have your error, feed it to the three opamp circuits I mentioned: one with just gain, another one- integrator, third- derivator. Add all three- the result should be used as the PWM command signal (remember?).

And you will have to tune the system, so use potentiometers around opamps.