Help doing a basic project.

I want to do a project that looks like the demo, but i have tried to do it modifying the demo but i have no clue. I am looking on how to do a conveyor with a sensor at the end. I want to make two sources, two cans with different heigts, one one (i would like to make them appear random, but that i will try to do it further along the project since i guess is harder). Then the ones that are high enough to make the sensor go on, to grap them and put them on a parallel conveyor. When a can touches the first conveyor, i want the conveyor to stop until it is picked up. 
later on, i would like to connect this to a plc siemens, so i would like to get started this project by projecting it on this goal (meaning to put the sensor as an output, something like that)
I am looking for help to put me in the path on completing the project because i do not know how to start and i have watched the tutorials. Any help is appreciated.
thank you