How do I start an essay on power relations as the theme?
This is a great topic, but as you can imagine, it is a very broad one. So, what you would need to do is narrow down the topic of power relations. In other words, you need a thesis. You can start by asking what aspect of power relations you want to explore. Let me give you some suggestions.
First, an interesting paper would be on the mechanisms of power relations. In other words, how does one person or group exercise power over another person or group. Some good postmodern theory can be helpful. I suggest you read people like Peter Berger and Michael Foucault. Here is a great quote by Foucault:
"Each society has its régime of truth, its ‘general politics’ of truth: that is, the types of discourses which it accepts and makes function as truth; the mechanisms and instances which enable one to distinguish true and false statements, the means by which each is sanctioned; the techniques and procedures accorded value in the acquisition of truth; the status of those who are charged with saying what counts as true."
Second, you can write about revolutions or what happens when those without power revolt. This is an interesting topic in view of what is happening around the world of late. Think about the Occupy Wall Street protests.
After you come up with a thesis, find some good supporting statements. Then you will be well on your way.