Arduino- which you can get at the maker shedhttp://www.makershed.com/ProductDetails.asp?ProductCode=MKSP4
Then you'll need a Ping UltraSonic Sensor which you'll find at most radioshack stores...http://www.radioshack.com/product/index.jsp?productId=2909789
A base and scrap metal or plastic that you have lying around.
Some extra wires.
Some cheap mounting tape from Walgreens or the dollar store.
Two Futaba S3003 servos.
Some lego wheels -4 of them ......or other small wheels.
Also a rechargeable battery. (The only reason is that the servos use a lot of energy.
Now your all set to go!