Your first step in funding graduate school is to complete the Free Application for Federal Student Aid (FAFSA). You cannot apply for, or receive, any financial aid from any institution of higher education without completing this form. Don't wait to be accepted to a graduate program to complete the FAFSA - apply early, while you are submitting your applications. Financial aid packages are awarded at same time as acceptance letters. If you wait to apply you'll lose your opportunity for aid.
The U.S. Department of education runs a variety of student financial assistance programs, including grants, and loans. Your eligibility for aid is determined by the information that you provide in the FAFSA. The graduate program and university will also use your FAFSA to determine your eligibility for scholarships, grants, and institutional aid. Learn more about the FAFSA and apply: http://www.fafsa.ed.gov/index.htm