I need help with a thesis statement about homelessness.

This question is really wide open at this point because it is not clear which direction you would like or need the thesis to go. It's possible that the paper is going to point to a cause for homelessness that needs to be addressed, or the paper might be arguing for a possible remedy. Regardless, the important thing to remember about a thesis statement is that it is going to make an argument that the paper will attempt to prove.
There are two general thesis formats that frequently work well. The first is a statement followed by a reason(s). This format requires the thesis to make a statement about something, and then it provides a few reasons of support/cause/effect/etc. For homelessness, this thesis might look something like the following statement: "Homelessness is bad for communities because it lowers property values and requires too much police attention."
The second thesis format is the point and counter-point format. This thesis begins with a dependent clause that is not your main argument. You are actually attempting to disprove this part. The dependent clause is followed by an independent clause that contains your main argument. This thesis might look like the following thesis statement: Although a community's homeless population is detrimental to property values, its detrimental effect on police forces is far more substantial.