Dates and deadlines
Every state has different priority deadlines for the FAFSA. I would suggest you look up the deadline for the state of your school.
For the 2013-2014 FAFSA, you will be using 2012 tax information. The 2013-2014 FAFSA will go out on January 1, 2013. By that date, nobody will have filed their 2012 taxes. Therefor you can submit your application using estimated income (in ways that the others above me have replied). Once your parents have filed taxes, which you should tell them to do as soon as they can in 2013, you can log back in to your FAFSA and make a "correction." When updating your tax information is important that you look at the help boxes on the right hand side. They tell you exactly what lines to look at on your parents taxes.