You didn't mention what part you're using so I don't know which physical pin you're trying to control, but as an example:
GPIOA_PDDR |= (1 << 17) //makes the pin an output by setting bit 17 of the data direction register
GPIOA_PDOR |= (1 << 17) //drives bit 17 high
GPIOA_PDOR &= ~(1 << 17) drives bit 17 low
If you're using it as an input:
GPIOA_PIDR &= ~(1 << 17) //enables input
if(GPIOA_PDIR & (1 << 17))
{
/*do something if bit 17 is high
}