Writing a thesis statement is actually the easy part. All a thesis statement consists of is one or two sentences in the introductory section of a paper or essay that declare the direction or anticipated findings that will follow. The thesis statement represents your objective. It can be argumentative or purely analytical, but it is short and concise and sets the tone for the rest of the paper.
A thesis statement on the general subject of homelessness is impractical in-and-of-itself. Nobody is pro-homelessness, partisan rhetoric notwithstanding. In order to develop a thesis statement that deals with the problem of homelessness, then, you will need to think about what precisely it is you hope to say. With a topic like homelessness, you're not arguing for or against it, obviously, but you can develop a statement that suggests a causal relationship between, say, government welfare policies and incidences or rates of homelessness, or between broader economic policies and poverty rates that contribute to homelessness. A perfectly legitimate statement could illuminate the relationship between substance abuse and/or mental health problems and homelessness. Many of the nation's homeless population do suffer from some form of mental illness, so a thesis statement could focus on that relationship. A perusal of the easily-available online literature on that topic could help focus the student's mind on a direction [See, for example, the scholarly article provided at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1350333/pdf/amjph00246-0033.pdf].
There has been a great deal of research conducted on homelessness over the years, and much of that material is available online. Once the student decides on a more focused topic than the generic "homelessness," it will be possible to develop a thesis statement with little difficulty.