The most commonly used function on arduino is digitalWrite(), the function used to pull an IO pin high or low. Lets start by taking a look at how we do this on an AVR.
First set the pin to an output as shown in the pinMode() tutorial.
The following code turns pin 5 of port b high:
PORTB |= (1<<PORTB5);
And to turn the output low:
PORTB &= ~(1<<PORTB5);
Simple!