Do you mean that you are needing to write a program for work or that you want to start learning PLCs so that you can get work in the field? Both are fine. If it is the first one, then MrDnesco's post is a great place to start. Flowcharts work well as well for understanding how your program operates. If it is a sequence that you must do, then you can write out each step of your sequence so you understand the order of everything before you start programming. Mapping your IO is also very important, and by that we mean list every single input or output your process has, wants, or needs. Then familiarize yourself with whatever set of instructions you have, and think of each rung as a goal. Think "I want this thing to turn on. I want it to turn on when ____. I want it to turn off when ____. I don't want it to on when ____. I don't wan't it to turn off when ____." That should give you a pretty okay set of instructions. Feel free to ask any questions you run into here!
If it is the second part, that you want to learn to get work, the book that PLCS.net is quite nice! also look into Hugh Jack's "Automating Manufacturing Systems with PLCs". Excellent book that is catered towards Allen Bradley instructions, but is workable for any PLC you can find.
Good luck either way!