Unless your assignment is different than you described (for example, if you were told to write about a smaller issue that is related to the nature vs. nurture debate), then taking a side pretty much tells you what your thesis is. Either you think the environment has more impact than genetics, or you think that genetics has more impact than the environment. So if you have already chosen the “nurture” side of the debate, then your thesis is “the environment plays a bigger role in a person’s personality” (although I might tidy it up to something like “the social environment in which a person is raised has more of an impact on someone’s personality than their genetics”).