Quick -- do you know what a FAFSA form is? FAFSA stands for Free Application for Federal Student Aid. If you're only going to bother with one financial aid form, this is the one. It's required by all colleges that offer financial aid, and is also mandatory for most federal student aid programs. For some federal plans, however -- including the Federal Family Education Loan, (FFEL), the Stafford Loan, and the Direct or FFEL Plus Loan -- you'll need to file additional forms.
You can get FAFSA forms in the fall at your youngster's high school, but you don't submit them until after Jan. 1. (Aim to submit them just after Jan. 1, as some funds are first-come, first-served.)
Once you submit the forms, you'll receive a Student Aid Report (SAR) that should be checked for accuracy. It will state your "Expected Family Contribution" (EFC), which is how much money you're expected to contribute toward your child's education. The schools you apply to will each get a copy of the results and will base some or all of their financial aid decisions on the information it contains.
For details on how and why you might fill out the FAFSA online, as well as all the information you ever wanted on the FAFSA form, drop by the Department of Education.