Omar - others may offer you better help that I am able to provide but here's a few thoughts:
1. I may be mistaken but if the load you are driving with PWM is inductive (a motor is inductive) then you'll probably want to keep the frequency relatively low -possibly limiting to a few hundred Hz.
2. Take a look at the datasheet for a 555 as there are some very good indications of how to determine the frequency, duty cycle - and that indicate some limits to the values of R1, R2, the capacitor, etc.