The Free Application for Federal Student Aid is here https://fafsa.ed.gov/
Advice on how to fill that FAFSA out is available here, also for free: https://www.edvisors.com/fafsa/book/direct/
Some private colleges also require the Profile Form, and information on that can be found here: https://student.collegeboard.org/css-financial-aid-profile
National Association for College Admission Counseling (NACAC)
The College Board SAT, ACT, APs, and My Big Future for a college search tool