In general, most but not all American college-age students and many older students are eligible for financial aid. If you still live at home, the government will take into account the income of your family (one or both parents). Otherwise, factors such as your income and your spouse's income may effect your eligibility. Before filling out the Free Application for Federal Student Aid (FAFSA) contact a counselor at a local college for help exploring your options.
Many but not all international-school American students may be eligible for financial aid. The best place to check is with the school itself. Call or email them and ask if the U.S. government typically funds American students that attend their campus. If they say yes, you can at least count on the possibility of financial aid. If they say no, then you might have to pick a different school or find another source of funding.
The simplest (although not the quickest or most direct) way to find out is through filling out a FAFSA online at the government fafsa website. Depending on when you fill the FAFSA out, it could take a couple months before you hear back about what government funds you're eligible for. The FAFSA will clarify what you personally can receive and how it relates to your school of choice. The FAFSA website notes that, "Foreign schools may or may not be able to receive your Student Aid Report (SAR) electronically. Check with the school selected on your FAFSA to see if they will need a paper copy of your SAR."
If you'd rather just find the Federal Code rather than fill out a FAFSA, you can go to the FAFSA website's directory, which will prompt you to select that you're searching for a Federal Code. Afterward you'll input that the school is foreign followed by its individual name. The website will then tell you if you're school is represented by a Federal Code and what that code is. Keep in mind you'll need to fill out a FAFSA to receive any financial aid, so knowing the school's Federal Code alone won't be much help.