Most Important Thing When Planning Application

As a programmer i'm not feeling in my own sphere when speaking a about application planning. It is not exactly my job - to plan. Programmer`s job is to implement the plans of product manager/designer/teamleader (and so far i'm only programmer).

Anyway, since planning is tightly connected  to programing, i think i must to have my own opinion on how those very important processes should be done correctly.  So here are my own conclusions on those topics

Point number one - when starting planning application it is very important to make clear (firstly for your self) - What this application should do , what problem it suppose solve? This point better to be formulated as short as possible

It is good to have the main application`s target as one sentence, for it will help to avoid following designs:

 (Motorcycle that initially planned to be a tank)

Also - to have a clear answer 'why i making this application' will make you understand if and when your goal is achived