"Adventure is just bad planning." [Roald Amundsen]
If you have to make a plan for an IT project, think BIG: What will the environment of your application look like ten years from now? Five years? One year? The scope of your timeframe as well as the scope of your application will define the range or the necessary depth of your plan.
A book which gives you a very good overall feeling:
The Mythical Man-Month, Frederick P. Brooks, Jr., ISBN 0-201-00650-2
If you develop together with five or more colleagues, consider using Microsoft Project.