We turn on the output bit (and light our LED) by using the same variable we used to read the input state:

PORTA = 1 ;

We can set the register to any value like this, but remember that we'll overwrite whatever was in the register before, so if you want to just alter the values of certain bits in the register, you'll need to be careful.

If you try setting pins that are set as outputs high, you won't actually set the state of that pin to high, you'll instead alter the state of the pullup register on that pin.