Researching the publishing market (books, magazines, trade publications) and finding out what type of writing is in demand is one of the most important factors in successful freelance writing. You need to know who wants the kind of writing you do. You also need to study the craft of writing. Take courses, read trade publications, and how to books and then write and write some more until you have got the best you can produce. Knowing how to approach an editor is crucial usually with a query letter as knows manuscript form and submission procedures. There is no guarantee you work will be accepted, but those writers who persist and work at improving their writing skills may eventually get published.