That is a very in depth question… see the link below.
But in a "large" nutshell writing the project plan or WBS has a some steps that are pre-requisite to this.. for the plan to be affective you must first do some homework and get down to the scope and deliverables of the project… simple going straight to the project plan will prove to be a counter productive task. You will also need to develop and get a grasp on your resources that will be involved in the project.

There are software that can help in this area.. just as Microsoft Project, Gannt Charts etc.