In order to argue this, you must make two major points.
First, you must assert that the fetus (up to whatever point in the pregnancy you want to argue that a woman has the right to abortion) is not a human life. You must assert that the fetus is instead something that is in a sense part of the woman's body.
Second, you must argue that all people have the fundamental right to care for and protect their own body. You can argue, for example, that we would never require a woman to get pregnant and that we would never (at least not since the ending of various forced sterilization programs in past decades) tell her that she may not get pregnant. These are choices that the woman must make about her own body and her own life.
If you assert that the fetus is not a separate human life, you can say that abortion is a fundamental right for a woman because it involves giving her control over her own body.