Create a C program that uses sensor to change LEDs(mini stoplight)

The sequence of LED is on the breadboard and have finished code for it. We have started the code for it, butt he only thing we need help with is programming a sensor into the circuit. The wiring itself is completely finished and a picture has been attached for it. The sensor itself has to change to the next LED within half a second(LED's go in the order: Green, Yellow, and Red it is supposed to work like a stoplight, where once the sensor is detected, it jumps to another LED. I have an attempted code for it, feel free to make the necessary adjustments. The end result should work like a stoplight