The FAFSA website encourages students to apply as soon as possible after January 1, but you will not be able to complete the application until you and your parents have finished your federal income tax returns.
Schools set their own financial aid deadlines, which might differ from federal and state deadlines. Corrections to the FAFSA can be made after the initial deadline has passed.
You can get an idea of how much financial aid you might get before you even start your FAFSA. The "FAFSA4caster" website gives students an "early estimate" of federal student aid.