When you must choose your own subject, you may immediately decide what you want to write about. A friend of yours may have been injured in an automobile accident caused by a drunk driver. You may decide to write about how beer and wine are advertised on television and to try to analyse the appeals that ad writers are making to young people. Occasionaly topics pop into your head,and you will find you can write well about them.
But most writers arrive at their topics by a slow, uneven process. You may want to write about religion and morals. But thousands of books have been written on that subject. You must bring it down to an essay you can manage in five or six pages. You want to your essay to be intersting , to grab attention but not to be vulgar. And if you write in general about religion and morals, you will probably not interest anyone. But suppose you write about the differences among Christians, Moslems, and Jews when it comes to the drinking of alcohol? Suppose you write about the problems of separation between church and state in this country, taking an issue such as prayer in the public schools of federal aid to church-supported schools or the teaching of religion in the public school? Suppose you write about the various constitutional issues involvled in the choice of textbooks for public schools? Find out what various religions groups have said on these subjects and you may have the basis for a fine paper.
You may want to write about physical exercise. But you can look at any large newsstand and see perhaps a dozen or more magzines covering various forms of exercise---bicycling, walking, jogging, weight-lifting, wrestling, and so on. How do you limit your topics so you can write about some aspects of exercise that will interst your readers? If you refuse to limit your topics and stay with making general ramblings about a general subject, your work wil be tedious. Don''t be satisfied with being tedious.
Sharpening your subject to make it more intersting may take much thought. You may find it easier to develop a specific topic if you take a series of steps to limit it progressively.