It may seem odd to list "thinking" as a part of the prewriting process, but this usually is the first step.
In the first few days after John is given his writing assignment, he thinks about his subject and what he might write about it. "Migrant Mother. What exactly is a 'migrant'? I'll look that up," John thinks as he is driving home from school: "The family sure looks poor. That's obvious. But is that the only point of the photo?" The next morning, while taking a shower, John continues to think about the photograph: "The title indicates that the woman is a mother, so those must be her children around her. Is the photographer trying to tell us something about what it means to be a mother?" John's questions lead him back to the photograph, and he decides that it is time to examine the subject of his essay more carefully.
The first step of the writing process can occur almost anywhere--while you are driving, while you are taking a shower, while you are mowing the lawn. The first step simply is to think about your subject.