The Free Application for Federal Student Aid (studentaid.gov) is the single most important form in this entire hub. Federal Pell Grants, work-study, and subsidized loans all flow from it, and most state and institutional aid requires it too.
File it even if you assume you will not qualify. Many need-based awards are calculated from your Student Aid Index rather than from Pell eligibility, so students who assume they earn too much regularly leave money behind.
It covers more than four-year degrees. Trade schools, technical colleges, and certificate programs are eligible too.
It opens around October 1 for the following academic year, and it uses tax information from two years prior, so no new paperwork is needed. Every contributor, meaning you and depending on your household a parent or spouse, needs their own studentaid.gov account. New accounts take a few days to verify against Social Security records, so create yours before the form opens.
File early. Some state and institutional aid is first-come, first-served.
It is always free. Never pay a third party to file it for you.
Sources: Federal Student Aid, studentaid.gov; Federal Student Aid Financial Aid Toolkit, financialaidtoolkit.ed.gov.