You will need to locate the Head Start office in your area and then fill out the application. You will need her immunization records, copy of her birth certificate and other information. Depending on your state/area, it may be income based as well. If that is the case, then you will have to provide pay check stubs too.
You'll have to find a school and fill out an app. Some require a registration fee and will ask about income depending where you live. Where I live its $241 a month for 4 days a week and 3 hours a day but they have a free program that you can apply for. It took me a few months to find a school and get my 4 yr old registered. Most fill up really fast so the sooner you do it the better.