www.fafsa.ed.gov or download a copy from www.ed.gov/offices/OPE/express.html. The former is the probably easiest of the two, and both electronic versions deliver the added bonus of automatic checking for common errors. Deadlines vary depending on the college. Many selective private schools require filing by February 1 in the winter preceding enrollment; deadlines at state universities and community colleges are generally later.
As federal forms go, the FAFSA is relatively simple. Annual income primarily determines whether a family qualifies for aid. Large households receive favored status, as do those with more than one student in college. Older parents get a break because they are closer to retirement and, in cases of divorce, the finances of the non-custodial parent are not included in the calculations. Family assets also play a role in determining aid eligibility, but the federal formula excludes equity in the family's primary residence and the value of life insurance policies and retirement plans. Income and assets in the student's name take a larger bite out of aid eligibility than those in the parents' name.
Ideally, parents should file this year's FAFSA on the basis of your recently completed tax forms. But with deadlines looming, it is better to file on the basis of estimates than to wait. (A chance for corrections will come later.) In the event of special circumstances, such as unusually high medical bills, make your appeal directly to the colleges for special consideration. Do not bother attaching anything to the FAFSA -- the people who get it are hired solely to crunch the numbers.