I cannot stress enough the importance of rigour around project plan tracking.
When you start a project you need to understand what it is that you are aiming to achieve at the end of your project. This will of course take the form of detailed information in a Project Initiation Document or Business Case, sometimes both.
You should also have detailed business requirements (for IT based projects) which detail exactly what the system will/should do when you’re finished. These will be used and validated during each of the project delivery phases, and especially in testing.