One of the classic electronic analogy to "Hello World" is to make an LED blink. Raspberry Pi has 8 General Purpose Input/Output pins( 4,17,18,21,22,23,24,25) which can be configured as input/output and turned on/off via software.In this Instructable I will be using python to control the LED, as coding in python is very simple yet it's very powerful. RPi.GPIO Python library is used to control the LED from Raspberry Pi.
Step 1: Hardware setup
Place the LED with a 270 ohm resistor in series with the GND and GPIO pin of Raspberry Pi (use a breadboard for hassle free connection). Always use a resistor with LEDs to limit the current, else you might end up with a burnt LED ;)
For complete details & schematics, please visit my blog :- http://www.rpiblog.com/2012/09/using-gpio-of-raspberry-pi-to-blink-led.html