You will do a main program loop (round robin multitasking) which keeps the checking sensor flags every few mseconds depending on your program size.
Make the PWM generated by interrupts also so that main program loop has better things to worry about.
You need to think about the design requirements before you start programming so that you know what to do.
So the robot will be standing still rotating its sensors till it finds the ball, then do you want the robot to move towards the ball or what?
If the ball light stops, do you want the robot to stop and go into search mode?
Mahmood