Prajna,
That's a bit like asking 'what are all the things to consider before implementing SAP ERP'. The answer is 'everything that will be required to support the business requirements'.
APO is a substantial system, with many modules. Most implementations only include a small subset of the modules that are available for implementation. It is impossible to answer such a question without knowing more about what functionality is to be implemented.
Typically, a project team will gather information from the appropriate businesspeople in an effort to develop the scope of the project. Only after the scope is known can you then begin thinking about what needs to be considered before implementation.
Best Regards,
DB49