Order of Writing
Most people begin by writing the introduction of an essay. This is a mistake.
At this point, you don't yet know your essay very well. You have barely started putting it together. Can you introduce somthing you don't know very well?
This is why I strongly recommend writing the introduction after you finish writing the body paragraphs.
The Introduction — Wait until Later
The introduction is not just the thesis statement. You need to begin an introduction with introduction techniques. These include anecdotes, interesting facts, quotations, questions, or disagreeing with someone else's beliefs. Most of all, introduction techniques must reflect upon and lead up to the main point of the essay.
However, you have not yet written that essay! Writing the paragraphs will lead you to better understand your topic. Writing the body paragraphs may lead to a change in your tone and/or organization for the essay.
Most of all, your essay does not exist until you have written the body paragraphs! How could you introduce it before it exists?
The Body Paragraphs
Instead, you should write the body paragraphs first. With the body paragraphs, you will probably want to write them in the order they will appear in the essay.
The reason for this is because it is important for the body paragraphs to work together. They must fit. One must flow smoothly into the other. If you do not write them in order, they might not flow well.
Therefore, you must not only write them in order, but you must pay careful attention to the relationship between the paragraphs.
When you are writing the second hlaf of the first body paragraph, you should already be thinking about the second body paragraph. How will I change ("segue") from this paragraph to the next? If the only connection is the subjunctive adverb (e.g., "second," "next," etc.), that is a weak connection.
If, for example, I am writing about my sister being clumsy, and my first body paragraph is about her being physically clumsy, and the second body paragraph is about her being socilally clumsy, I must consider this while writing.
Perhaps the second example of my first body paragraph is about her tripping and falling, dropping the groceries when she comes into the house. Knowing that I will soon be writing about social clumsiness, I might want to emphasize that in my current story. Perhaps my sister, when she fell, shouted out a lot of inappropriate language. This could be a social error (and possibly funny!), but also could mark the beginning of the change to social awkwardness.
Then, when I begin my paragraph on social clumsiness, I might want to begin with an example that is related to saying the wrong thing at the wrong time. I could use a transition technique in the topic sentence which marks this connection. "My family are not the only people she offends; her social life is full of errors cause by her graceless nature."
In this way, you get to have very good coherence: there is a smooth link between one paragraph and another. You end one paragraph with a story that has a link to the topic of the next paragraph, and use the topic sentence to express that connection.
If you use this technique, you may decide to change the order of body paragraphs when you write, as you notice possible paragraph-to-paragraph connections which you had not seen before.
Back to the Beginning
After you have finished your first draft of the body paragraphs, you can then return to your introduction. Now that you know exactly what the body of the essay is, you can better decide what will effectively introduce the body.
There is another benefit to waiting until now: leftovers. After you finish writing the body paragraphs, you have finished using the ideas from your brainstorming. However, since you came up with 12 or more example stories, it is very possible that one of the brainstormed examples is interesting and powerful, but did not fit into the body paragraphs.
Perhaps there is an excellent or entertaining story, but it did not fit together with any other example to make a topic sentence category. Read through your remaining examples, and see if one of them, in some form, might make a good anecdote for your introduction!
You may want to do a little more brainstorming, in fact. Look at all of your examples together. Do they remind you of anything? Can you summarize the body paragraphs in a few words? Do a Google Search with these words, then add the search term "quotation" or "interesting fact."
I did this with the word "clumsiness," and came up with the English expression, "I am all thumbs today." I also found a cute expression: "I'm not clumsy. The floor just hates me." The expression "I'm all thumbs" is just an English expression, and needs no citation. The "floor just hates me" idea might need a citation, however. In that case, you had best ask your teacher what is necessary; it would depend greatly upon the specific case.
My point is, writing the introduction can be much more effective and even perhaps easier after you finish writing the body paragraphs. After finishing the introduction, you can then go on to the conclusion, which is usually more related to the introduction than to the body paragraphs.