SeanLee,
You should download a PLC software program, and then practice writing programs. Usually you can find a free one, or at least a 30-day trial offer, or a "starter kit". The starter kits have a small PLC, the programming software, connection cable, and user manuals. Look at Omron ZEN, AutomationDirect.com, Siemens, and Allen-Bradley (Rockwell) Micrologix PLC's.