First, you will need to come up with a thesis statement. However, to come up with one, you will need to decide what the focus of your essay will be. If your instructor has given you the choice to write about anything you like regarding female homelessness in Seattle, you may be in an enviable position. However, with great choice comes great responsibility. What will you focus on?
Will you write about the reasons for female homelessness? If so, your thesis statement will likely highlight the root causes of female homelessness and ways to combat it. If you decide to focus on what the city of Seattle is doing for homeless women, your thesis statement will highlight the effectiveness of social programs that benefit homeless women in Seattle. From another angle, you may wish to connect the problem of female homelessness to greater problems such as sex trafficking and the drug trade. Your thesis will then highlight what the women themselves are doing to escape the problem of homelessness. Sometimes, the women are drawn into the sex or trafficking trade out of desperation.
For some help in solidifying your own positions and thesis ideas, try some reading from the links below. Perhaps not all of them will be relevant to you, but many of them may provide you a basis for formulating your own ideas.
The Seattle Government's Ten Year Plan To End Homelessness in Seattle
Seattle Public Lands For Use As Encampment Sites for the Homeless.