When you are making a thesis statement to put in the introduction of your paper you want to take a clear stance on the topic you are writng about in one to two sentences. You also want to take an verbally impartial stance, this means that you want to avoid emotionally inflamatory language.
If you are supporting a woman's right to abortion and are taking a prostance choice you will want to look at what factors influence this decision and support this statement.
For example: The presence of a woman's right to choose her reproductive rights is positvely correlated with higher level's of education and income, while there is a negative correlation with women who are not permitted reproductive choice and education levels.
This statement shows what your topic is (prochoice) and offers why this is a good idea (education and income level). You then want to support these concepts within the topics of your paper and expand on how prochoice is helpful and provide example of how removing this option from women can be detrimental.
[Please note: this answer is intended to be objective and answer the question, not to take any specific stance on the issue.]
This is a controversial topic, so there can be many possible theses that would be appropriate, from many different viewpoints. One good area to start is to identify personal beliefs on whether abortion is a necessary medical procedure, or if it is unnecessary given the availability of birth control and women's rights. For example, one good argument is that regardless of moral or other objections, abortion should be legal for the safety of the woman -- that is, if a woman is going to get an abortion, it is better to have a safe legal procedure rather than a potentially unsafe procedure by an unqualified, illegal institution. For this argument, the thesis statement might read:
Abortion should be legal and medically-supported; otherwise, unsafe procedures will cause more fatalities in women, thus undermining the moral objection of saving human lives.
This is only one possible view; other arguments might focus on the right of a woman to make decisions about her own body regardless of the effect on others, or the legal/biological status of a fetus and where "life" is considered to start. One question is whether the right-to-choose supercedes the potential right of a fetus; this, naturally, depends on legal or personally-ethical definitions. Other theses may focus on issues or rape or incest, and otherwise explicitly unwanted or forced pregnancies; in these cases, the thesis should probably focus on a woman's personal rights rather than on medical or ethical issues.