Power supply circuit for powering SG90 servo. How to handle high current requirements?
Don't reinvent the wheel, just get a voltage regulator. Digikey sells efficient switching stepdown regulators that will output 5V @several amps (like this one). You can probably get one for &lt; $10, all parts included.

For your circuit, I would be concerned about R3. If you want to output 0.1A, the voltage drop across a 100 ohm resistor is going to be 10V. 12V - 10V = 2V. Also, there's probably going to be other problems that I haven't foreseen yet (I'm not an expert on voltage regulator design).