Here's how I would start:
Forget about the PLC aspect for bit. Start by fully defining your system (conveyor, traffic lights, elevator, whatever). Your definition should include:
1) A complete narrative of how it should work - specifically, what the output conditions will be, every possible input condition, and the general logic that ties the two together.
2) Make a list of the required input and output devices.
3) A flowchart or pseudocode development of what you're trying to do.
I am a relative novice so the gurus here may be able to expand on this list or point you to a thread in which it's already been discussed.
Once you have that information laid out, it's then a matter of coding your project which means you'll have to learn the details of whichever package you use.