Fortunately, you have chosen a topic that immediately resonates with many people, most of whom are already primed for a reason to detest the practice of human trafficking. The spectrum of people who might support the idea of human trafficking is mercifully small, which makes your task in an argument essay more focused: that is, you can concentrate on an argument that assumes the evils of human trafficking and explore the fact that it is still, despite its already reprehensible nature, an evil that needs to be eradicated.
First, and most important, any argument essay begins with a fair amount of research. You need to read as much as you can (given your time constraints) about human trafficking. Why does it exist? Who are most likely to be the victims? Are certain countries more likely to encourage it? What are the personal and societal consequences of such trafficking? This research becomes the framework of your thinking about the topic and will guide you to the points you want to focus on given the length requirements of your argument essay. You cannot cover every element adequately, but you can explore several elements adequately to support your thesis.
Your introductory paragraph will tell your reader what your subject is, inform them what your view of that subject is, and, most importantly, encourage the reader to read the rest of the essay. How do you accomplish that? One of the most effective ways to begin an argument, especially on a topic such as human trafficking, is to gather as many statistics about human trafficking as you can and begin the essay with a recitation of those statistics. Where does human trafficking occur? What types of people are victims of human trafficking—what gender, what age, what background? Is human trafficking a problem worldwide, or is it limited to specific countries? The advantage of using statistics from credible sources (.gov, .edu, or .org sources) is that they are not arguable—that is, a reasonable person will accept the validity of statistics from credible sources. Facts and figures, by their nature, tend to hook a reader into continuing to read.
A second and equally effective method to hook your reader is to construct a narrative that focuses on a typical person who is a victim of human trafficking. In other words, you briefly tell that person's story and point out that this is a representative experience for those who are victims of human trafficking, using as much detail of that experience as possible. Readers, even those who might not be particularly interested in the topic, are drawn into a subject if they see themselves or someone they know in a particular situation. The main point is to motivate the reader to read further.
At the end of the introductory paragraph, or at the beginning of the second paragraph, you need to state your thesis—your argument about human trafficking. This must be an affirmative statement, not a question. In simple terms, your thesis should not be "Is human trafficking an evil that must be eradicated?" A workable thesis might be something along the lines of "Human trafficking must be stopped wherever it exists because it is a modern form of slavery."
Your thesis statement, whether it concludes the opening paragraph or begins the second, should be as detailed as possible. If you have decided, for example, that there are four elements of human trafficking that you want to discuss, then the thesis should briefly introduce each of those elements. This will be, in effect, your road map to the essay itself and will tell the reader what to expect.
With this particular topic, you will not have much opposition to your point of view—unless, of course, you argue that human trafficking is a good thing—but you can create a kind of opposition by noting in your thesis that you recognize that human trafficking is a very profitable business in some areas of the world. You will argue, however, that the economics doesn't justify the human suffering, so that your reader understands that you are not unaware of the economic "benefits" of human trafficking.