Typically you start by looking at a specific idea that you want to convey. For example, you may want to talk about how you feel about war. I generally write down single words that describe what I feel like hate, rage, bloody.... Then I may move on into phrases. As the words and phrases grow then I look at the range of my emotions about the topic and begin to write it all out in first person. Once it's down on paper you need to ask someone to read it with a careful eye and see what grabs them.