Once you have fleshed out each of your body paragraphs, one for each main point, you are ready to continue.
If your main idea is "reduces freeway congestion," you might say this:
Public transportation reduces freeway congestion.
Commuters appreciate the cost savings of taking public transportation rather than driving.
Less driving time means less maintenance expense, such as oil changes.
Of course, less driving time means savings on gasoline as well.
In many cases, these savings amount to more than the cost of riding public transportation.
Your essay lacks only two paragraphs now: the introduction and the conclusion. These paragraphs will give the reader a point of entry to and a point of exit from your essay.
The introduction should be designed to attract the reader's attention and give her an idea of the essay's focus. Begin with an attention grabber. The attention grabber you use is up to you, but here are some ideas:
Startling information. This information must be true and verifiable, and it doesn't need to be totally new to your readers. It could simply be a pertinent fact that explicitly illustrates the point you wish to make. If you use a piece of startling information, follow it with a sentence or two of elaboration.
Anecdote. An anecdote is a story that illustrates a point. Be sure your anecdote is short, to the point, and relevant to your topic. This can be a very effective opener for your essay, but use it carefully.
Dialogue. An appropriate dialogue does not have to identify the speakers, but the reader must understand the point you are trying to convey. Use only two or three exchanges between speakers to make your point. Follow dialogue with a sentence or two of elaboration.
Summary Information. A few sentences explaining your topic in general terms can lead the reader gently to your thesis. Each sentence should become gradually more specific, until you reach your thesis.
If the attention grabber was only a sentence or two, add one or two more sentences that will lead the reader from your opening to your thesis statement.
Finish the paragraph with your thesis statement.
The conclusion brings closure to the reader, summing up your points or providing a final perspective on your topic. All the conclusion needs is three or four strong sentences which do not need to follow any set formula. Simply review the main points (being careful not to restate them exactly) or briefly describe your feelings about the topic. Even an anecdote can end your essay in a useful way. The introduction and conclusion complete the paragraphs of your essay.