The following activities will help you get acquaintated with the Arduino.
The activities get a little trickier as you progress down the list.
Introduction to the Arduino
Arduino and a Flashing LED
Arduino and a 7 Segment LED
Analog vs Digital in Electronics
Arduino, Buttons and Fading an LED
Arduino and the Tilt Switch
Arduino and the Photoresistor
Arduino and the Potentiometer
Arduino and Traffic Lights