First, make a list of subjects for which you are either very interested or already know a great deal about. Wait. Back-up one minute. Before you make this list of subjects you've probably pondered about for a long time, take a new piece of paper or sit at you computer with a blank screen. Set a timer for 5, 10, or 15 minutes and start writing as fast as you can. List every subject that pops into your head.
Relax and let your mind wander on this, but write quickly letting anything pop into your mind that will. Don't hesitate. Don't analyze. When the timer goes off, stop writing. Doing this exercise brings to mind subjects you might never have thought about because people get so focused on one or two areas that everything else gets blocked out.
Go through your list and cross off everything you know absolutely that you would never want to write about. Write "maybe" next to things that seem to spark an interest. Write "yes" next to things you know you want to start writing about immediately. Of these "yeses," pick four or five things you are most passionate about and circle them.
With these 4 or 5 circled subjects in mind, do a very quick perusing in the library or online to see if there is much current information about each of these subjects. If you've been reading many recent articles and news items or research on one of the subjects, then skip this step for that subject. Don't take the time to do any research; all you are doing is finding out if there is current information about your subjects. Make notation if all you find is very old information. In today's world of fast paced media, information that is only a couple of years old might be out of date and hardly useful. If information you are finding is current, then you'll know these are subjects you can focus on. Write "wait" next to any subject for which you can't find recent or up-to-date information.
Don't start writing yet.
If you are using these ideas as magazine articles or online articles, then you need to know which magazines or ezines to target each subject towards. For sake of making things more easily understood, let's just say that you have decided to write about parenting a new baby. Uh, this is too broad. Decide whether you are going to write about sleeping and napping techniques for the new baby---or how to get sleep as a parent of a new baby. You might write about feeding the new baby---breast feeding pros and cons, formula pros and cons, types of bottles (plastic versus glass) and their pros and cons, feeding baby home-made baby food or bought baby food, when to start various foods and how, types of toys and educational recordings for babies, sound monitors and/or visual monitor pros and cons. Some people don't know that visual and sound monitors can switch signals, and you can possibly be seeing the neighbor's baby on your monitor instead of your own in your own house.
It should be easy to see how to narrow the "parenting a new baby" subject. No matter what your subject, break it down into smaller more narrowly defined areas. Use one area if you are writing an article. If you are writing a book, then these narrowed subjects can be your chapters.
After the subject is narrowed, research Writers Market to see which publications take your type of writing. Read at least two or three current issues to determine if the tone of the publication is friendly, conversational, authoritative, scientific, etc. Does it use personal stories? Are things written in first person or third person? Does most articles have quotes from experts in the field? Does it use dialog?
Only after you've determined the answers to the above things should you write your query letters to each targeted publication. Don't write the articles. Write the query letters and make the opening of the query letter so enticing that anyone reading it can not possibly put it down without finding out more about what you have to say. And, . . .don't forget to personalize each query to the right publication.