Follow the complete library example, save the key value it is returned when a key is pressed
//add this
byte new_key;
void setup(){
// whatever you had
}
void loop(){
char key = keypad.getKey();
if (key != NO_KEY){
new_key = key; //new_key is only updated when a key is pressed
}
//then add your
//switch (new_key),
//or the slower if (key == '1') etc.
}