The basics:
void loop(){
if (digitalRead(pushButton)==LOW){ // pin with internal pullup resistor, button closes to Gnd
ledState = ledState +1;
if (ledState == 4){ledState = 0;} // reset to 1st state
switch (ledState){
case 0:
digitalWrite (redLed, LOW); // LEDs with anode to pin, cathode to resistor to Gnd
digitalWrite (yellowLed, LOW);
digitalWrite (greenLed, LOW);
break;
case 1:
digitalWrite (redLed, LOW);
digitalWrite (yellowLed, LOW);
digitalWrite (greenLed, HIGH);
break;
case 2:
digitalWrite (redLed, LOW);
digitalWrite (yellowLed, HIGH);
digitalWrite (greenLed, LOW);
break;
case 3:
digitalWrite (redLed, LOW);
digitalWrite (yellowLed, HIGH);
digitalWrite (greenLed, HIGH);
break;
} // end switch:case
} // end detect button push
delay (100); // pause for button debounce
} // end loop
Leave it to you to assign signals to pins, declare variables,
set up pinModes, etc.