If you are writing a persuasive essay (an argument) against abortion, your first task is to narrow the topic to fit the space you have available. As a first step, I suggest you look at a website called Pro-Con.org (see link below), which sets out arguments for and against many current and controversial issues, including abortion. Based on what you see there, and your own view on abortion, you can formulate a thesis. In your case, because you are writing against abortion, you will focus on those arguments against abortion. The thesis statement, which will guide your essay, must be a declarative statement--that is, it must state your position directly and forcefully, not in the form of a question. The best thesis statements incorporate a general opposition argument, like this: "Although many people argue that free access to abortion services helps to keep unwanted births at a minimum, abortions have a much greater negative effect on society because [and here you state the reasons you will argue against abortion]."
Many teachers ask you to put the thesis statement as the first sentence of the first paragraph. I suggest. however, that you place it at the end of the first paragraph, a technique that allows you to hook your reader's interest by using facts and statistics that support your overall argument. Open the first paragraph with a list of statistics that overwhelms the reader: for example, statistics related to the number of unwanted births; the cost to society of raising an unwanted child; the financial and psychological cost to the birth parents, especially if they are below the age of 21. Using statistics at the beginning is effective because statistics, if they are properly derived, cannot be argued with. Someone who opposes you view has to argue other reasons for supporting abortions.
During your research, you will have identified a number of strong arguments against abortion. Keeping in mind that abortion is a very broad topic, narrow your focus so that you can argue a few topics thoroughly rather than many topics shallowly. Depending upon the assignment, you may have room to discuss only three or four arguments against abortion and, as you will see from looking at a site like Pro-Con.org, there are too many to handle in that space. Your essay's body paragraphs will each focus one specific argument, and you will have as many paragraphs as you have arguments.
As an argument strategy, in addition to laying out your own arguments against abortion, you must acknowledge that two or three strong arguments against your point of view exist. That can be handled in an early paragraph and will convince your readers that you understand the complexity of the topic and, more important, that you recognize that reasonable people can have radically different views on the same topic, especially one like abortion that appeals to emotions, religious beliefs, and to belief systems that are not religion-based.
When you write a good essay, you will often discover that, as you write, you may actually change your view of the topic slightly--this happens all the time to experienced writers and is a natural process. In that case, you may actually modify your thesis to accommodate those changes in your view, so think of the thesis statement as a work-in-progress, not set in stone.