Start with a "hook." If your reader is interested in your introduction, he will want to keep reading the rest of your paper. If he is bored by the introduction, he will want to put it down. Just because your college professors (or admissions officers) have to read your whole essay does not mean they want to; if you can hook their interest in the beginning, you stand a better chance of getting them to read your essay with objectivity. According to Hamilton College's Writing Center, "a well-constructed introductory paragraph immediately captures the interest of your reader." If it takes you too long to do that, the interest will be lost.
A solid introduction should include a unique thesis statement that is specific about your topic (don't just say something is good or bad; explain what is good or bad about it or in what ways it is good or bad). Give your reader a glimpse into the rest of your essay without summarizing the whole thing up front, and invite him to read more.
Build your persuasive argument in the body of the essay. Your essay should contain at least three middle paragraphs, each focusing on a specific argument that proves your point. For example, if you wanted to persuade your readers that peanut butter and jelly sandwiches are the only food they ever need to eat, your supporting paragraphs might include information on the ideal taste combinations of peanut butter and jelly, the nutritional value of the sandwiches, and the ease and portability of eating them in any situation. Your body paragraphs are where you will sell your argument to your reader and make her understand your point of view (and, hopefully, agree with it).
According to Dartmouth College's writing guidelines for English 24, "the main thread of a good paper is analytic: try not to let description or summary usurp the place of analysis." When you make your supporting arguments, try to get straight to the point and list facts more than opinions or explanations. This will give your paper the substance it needs to earn a good grade.
Finish your essay with a solid concluding paragraph. A concluding paragraph should not merely summarize the body of the paper. If it did that, no one would need to read the rest of the essay; they could just skip to the conclusion every time. It is equally important, however, not to introduce new information in your conclusion. New information is for your supporting paragraphs; the concluding paragraph is your chance to drive your point home (like a closing argument an attorney makes in court). In the conclusion, you should explain the significance or importance of your argument as well as any real-world applications it might have. As Dartmouth College puts it, "a paper should get somewhere, should conclude by placing the matter discussed into some larger context." Doing this will leave your reader with a positive lasting impression of your persuasive argument. Failing to do so may leave them with a negative lasting impression, which can lead to a bad grade.