Hi, As you may already realize, Head Start is a national program for low-income families, with local centers around the United States. Here's a fact sheet about Head Start from the National Head Start Association:
  
http://www.nhsa.org/files/static_page_files/A97D606C-1D09-3519-AD7E2646D5EA7CDB/08FactSheetHSBasics.pdf
  
And here is the Head Start locator on the U.S. Department of Health and Human Services website, so you can find a center near you:
http://eclkc.ohs.acf.hhs.gov/hslc/HeadStartOffices
  
If your family is not qualified to participate in the Head Start program, some preschools offer scholarships. You can contact potential preschools in your area and ask about their financial assistance offerings, or here's a video about scholarship money for preschool: http://www.ehow.com/video_4790646_free-scholarship-money-preschool.html
  
I'm also including a link below to a Preschool Readiness info center Education.com provides, with a variety of articles relevant to starting preschool.
  
Good luck - I hope you are able to a find a great preschool for your child.