Read, write, read, write, read, write, and read, read, and read some more. Learn by trial and error. Most of all, write because you want to write, and not because some writer you read about actually made money by writing. It happens to a very, very few.
Be open to criticism, and don't rely on your friends and family to give you an honest feedback. Stranger will always be more honest with you. And when you do receive criticism (and you will, no matter how good you think you are), don't take it personally but learn from it instead.