One example of a good thesis for an argumentative essay on the abortion debate might be, "Abortions should be legal in cases of rape, incest or endangerment of the mother's life." Another might be, "Abortion should never be legal, regardless of the circumstances." In either case, the writer's position on the topic is made clear.
After writing a clear thesis statement, it is important to provide sound arguments that support the main idea. In the case of the first thesis statement above, some supporting arguments might include statements such as, "While a fetus is a living being with the same right to thrive as any other person, there are exceptions when the life and well-being of the mother should take precedence." Including data on how rape and incest traumatize both mothers and their children strengthens that assertion.
In the case of the second thesis above, some supporting arguments might include, "Because a fetus is a viable, living human being, abortion under any circumstance is murder." To back up that argument, the writer should provide a definition of the word "murder" and include medical information that shows the fetus is almost immediately an autonomous being after conception. It is also important to provide reasons why the mother's life should not be valued over that of the fetus.