Stuff you will need....most of it is found quite cheap on ebay...
HC-SR04 sonar sensor Link
PC Pro Mini Module Atmega168 5V 16M For Arduino Compatible With Nano link
Any old servo you have lying around
Wires or Servo extension cables
FTDI USB TTL serial board adapter module to program the Arduino board link
Find your favourite seller, these are just the first ones that I came across.
Here is how I connected it all together.