//This program is very simple ...... if you are having trouble to understand ......just dry run the loop() for four different states of the "currentButtonState" and "lastButtonState" 
#define switchPin 8        // Just the switch definations
#define ledPin 13        
boolean lastButtonState = LOW;    
boolean currentButtonState = LOW;   // Used in the debounce function
boolean ledOn = false;

void setup()
{
  pinMode(switchPin,OUTPUT);
  pinMode(ledPin,OUTPUT);
}

boolean debounce(boolean last)            // This function works the magic...
{
  boolean current = digitalRead(switchPin);
  if (last != current)
  {
  current = digitalRead(switchPin);
  }
  return current;
}
void loop()     // Just dry run this function with four combination values of currentButtonState and lastButtonState
{
  currentButtonState = debounce(lastButtonState);
  if(currentButtonState == HIGH && lastButtonState == LOW)  // this means someone has just pressed the switch
  {
    ledOn = !ledOn;
  }
  lastButtonState = currentButtonState;
  digitalWrite(ledPin, ledOn);
}