After the introduction, Myrtle will want to write three paragraphs that, collectively, will make up the body of the essay. In each paragraph, she'll want to focus on one reason or counter, and include evidence to support it. For example, she might want to write one paragraph on the idea that she could study at the library for longer, another paragraph on how she's older and more responsible than she used to be, and therefore deserves a later curfew, and another paragraph on the counter that an earlier curfew does not mean that she'll get more sleep.
Perhaps it is helpful to think of an essay in terms of a conversation or debate with a classmate. If I were to discuss the cause of World War II and its current effect on those who lived through the tumultuous time, there would be a beginning, middle, and end to the conversation. In fact, if I were to end the argument in the middle of my second point, questions would arise concerning the current effects on those who lived through the conflict. Therefore, the argumentative essay must be complete, and logically so, leaving no doubt as to its intent or argument.