This uses an L293D chip as a motor controller

http://luckylarry.co.uk/arduino-projects/control-a-dc-motor-with-arduino-and-l293d-chip/

http://itp.nyu.edu/physcomp/Labs/DCMotorControl

int switchPin = 2;    // switch input
int motor1Pin1 = 3;    // pin 2 on L293D
int motor1Pin2 = 4;    // pin 7 on L293D
int enablePin = 9;    // pin 1 on L293D

void setup() {
// set the switch as an input:
pinMode(switchPin, INPUT);

// set all the other pins you're using as outputs:
pinMode(motor1Pin1, OUTPUT);
pinMode(motor1Pin2, OUTPUT);
pinMode(enablePin, OUTPUT);

// set enablePin high so that motor can turn on:
digitalWrite(enablePin, HIGH);
}

void loop() {
// if the switch is high, motor will turn on one direction:
if (digitalRead(switchPin) == HIGH) {
    digitalWrite(motor1Pin1, LOW);   // set pin 2 on L293D low
    digitalWrite(motor1Pin2, HIGH); // set pin 7 on L293D high
}
// if the switch is low, motor will turn in the opposite direction:
else {
    digitalWrite(motor1Pin1, HIGH); // set pin 2 on L293D high
    digitalWrite(motor1Pin2, LOW);   // set pin 7 on L293D low
}
}

Controlling Motors