For seniors applying to college, the Free Application for
Federal Student Aid (FAFSA) should be submitted as soon
as possible after January 1, 2013. But
the FAFSA asks for specific numbers from your 2013
federal income tax return. How do you
complete the FAFSA if your return won’t be completed yet? Use estimated numbers, then log into your FAFSA later when you complete your taxes. Whatever you do, don't wait to file the FAFSA.
Finaid.org’s Mark Kantrowitz gives more detail here.