[development] how to start a drupal like project

I want to write my own open source web application and make it as flexible and usable as drupal. I have no experience with a project this size. How do I go about it ? What do I need to do first and how do I plan the project ? I dont know where to start. I need to know the core technical side of project planning (not the hosting, cvs, etc..) data structures file system layouts theme, modules support, etc..