Well, first of all you need an idea. You should know what the beginning and the end is first. If you dont know the beginning then think of the end and figure out how someone can get to that point. if you have a beginning but no end, think about what you want your character to go through.
As far as character is concerned, you should have a character profile that details everything about your character from birth. Even the most mundane of things should be thought of, like what their favourite drink is.
Once you have character and plot, write your story the best you can. You improve through hard work and advice from others as you go along.