Create a business plan. You'll start getting clients by contacting realtors who handle foreclosure listings, banks (the acquiring institutions of failed banks who have "inherited" tons of foreclosures -- you can find them on the FDIC's website), and larger property preservation companies, who have listings all over the United States.
I could go on and on. Visit the industry link below that has a plethora of information that will guide you in starting your foreclosure cleanup business. FREE info; more than enough to get you started on the right track.
Many wishes of success to you!
Cassandra Black, Consultant & CEO, Foreclosure Cleanup, LLC