Warped front disks is the most common cause (you will feel it
through the steering and brake pedal). Other less common causes
(usually just shimmy in the wheel not the brake pedal)  are worn
tie rod ends, worn universal joint in steering rack, air in
power steering fluid (leak), worn suspension bushes, incorrect
toe in, loose wheel bearings and worn steering pump.

most likely warped rotors, take the car to brake place and get them to give
you rotors a fresh cut