In order to compose an effective essay, you want to be sure that you have laid out your "blueprint" thoroughly. That is, you need to outline what you are going to write. First, you want to state concretely what your particular "American Dream" is in your thesis. For instance, you could write,
My American Dream includes three ideals: ______, ________, and _______ . The three parts of your thesis statement can each be used as topic sentences of the body of the essay. For example, if your first ideal were success, you can write a topic sentence about succes, such as
One ideal that is part of my personal American Dream is success. There are different ways to define success, but for me it means -----
Then, you will add supporting details with explanation, and write a reworded topic sentence. After this, you begin your second paragraph similarly with a topic sentence that you support with details.
Finally, you write a concluding paragraph which sums up the three points that you have made.
This is a good question. If you define the American dream as being able to do anything, then you can approach your body paragraphs in many different ways. Let me give you some suggestions.
First, you can give a few examples of people who did accomplish something great. Your point would be most powerful, if this person accomplished something great against all odds. For example, Steve Jobs came from a poor family and look what he created - Apple, one of the greatest companies of all time.
Second, you might want to speak of the hardships in other countries where it is not possible to dream or to have a passion, let alone do something great.