Founds answer is not exactly correct. While you can't finalize the FAFSA form, you can submit estimates of what you anticipate the amounts will be. The best way to do that is use the figures from the 2006 return. When the 2007 taxes are complete, you can go back into the FAFSA and make changes were there were significant difference.
Just make sure you get it in on time. also, copy the schools to which you are applying and ask if they have any alternative loan programs. My son goes to school in connecticut. The state has a financial aid program supported by the use of municipal bond issues. It's available to all Connecticut residents and all students attending school in Connecticut, regardless of the students state of residence. The interest rate is exceptionally low.
Ask the Financial Aid office at each school to which you are applying whether they have a list of preferred lending providers.
Also, almost every college will allow your parents to set up a payment program year to year, through which they can pay all or a part of the tuition, room and board over a 10 month period without any interest payment at all.
Ask questions and keep asking the questions until you get the answers you're looking for.