First thing you need to do is write out everything the program has to do -- schetch out the screens, menus, and describe the functions that it must have and what they are to do. Only after you have finished that can you begin coding it.
Is this stock as in what is in a store, or is it stock as in "1000 shares of Microsoft"? These are VERY different domains, and the solutions are very different (speaking as someone who has developed both sorts of systems).
That said, read what AncientDragon wrote. That is the basis of either sort of system.