Do you have to write all four? Or can you choose just one?
Either way, the first thing to do is to identify what you want to say. For "I have a dream," you could talk about your own dreams or, I suppose, Martin Luther King, Jr.'s "I have a dream" speech and what it means to you. If you were to write about "families now and in the past," you would start by listing what you think are the differences, if any, between families now and in the past.
Only after you had decided on what you will say can you begin to organize your ideas into some logical order and come up with a thesis statement that states your main point. Only then you can begin to write.
Simone, EssayForum.com