What would be a good thesis statement for social welfare?

It depends on your stance on the subject. If you are against welfare you should say something like "Welfare is a negative aspect of the governments social programs due to the obvious problems of... blah blah blah" whereas if you are for it you should go with something like, "Welfare is a great program that assists many and benefits society as a whole because of... blah blah blah."

You might have a more specific idea for the paper, so lead with that in the thesis. The thesis is really just the main arching idea of the entire paper summarized in a sentence you put in the first paragraph.

actuallyy, a thesis statement should not have an opinion in it, and its suppose to like tell pretty much what your talking about through out the report so if it was about welfare and you said

welfare was bad because .... .... ....

and like say reasons why it was bad and you should explain about the reasons with more detail in the report.