The FAFSA application opens on October 1. You should fill it out in the fall of your child's senior year of high school. Check college websites and submit the FAFSA before each school's priority financial aid deadline. Your state also has a FAFSA submission deadline, which you can look up on fafsa.ed.gov.
To get started, you and your child should go to FAFSA.gov and get FSA IDs—usernames and passwords that you use to log in to the site and submit the FAFSA. You and your child will each need your own FSA ID.
You'll need to fill out the FAFSA every year.