How does a writer select or set the central purpose of his writing before he starts to develop the characters, plot and story. I am learning how...

This is tricky, because sometimes you will have a purpose before you create your characters, but sometimes, your characters will dictate your purpose.  One helpful technique that I have used and teach my students to use, is creating a specific character and giving that character some concrete objects as part of his or her essence. For instance, say you are really not sure how to approach a new story, but you want to play around with different characters and see what comes up. You can create two characters that have specific identities and have them converse and see where it takes you.

Say you choose to create a male and a female. Give them names, height, weight, age, professions, likes, dislikes, and a few objects that are specific to each of them. Maybe one has a pet. Maybe one wears bifocals. Maybe one smokes cigars. You can have fun playing around with this, and it will loosen you up to be open to new ideas.

There's no "right" way to approach writing fiction. You can even use your technical writing skills to help you be more precise or explain details. Sometimes, when you create a character, he/she takes on a life of his/her own and, before you are aware of it, you are in his/her story.

In fact, it may happen that what you begin with turns into something entirely different than what you had originally planned to write. Don't get too attached to it, and let if proceed for a while, and remember, we often have to write a lot of words to get to where we are going.