College financial aid is always based on annual income for the prior year. So for the Sept. 2013 - May 2104 school year, aid is based on your 2012 income. FAFSA can be filled out beginning in January, since until Dec. 31 of 2012 your income is not final.
Your parent(s), guardian, or whatever and you can fill it out as soon as you have all of the W-2's and other information, or if income is fairly predictable do an estimated FAFSA if the forms are delayed for some reason - and then update the FAFSA when those forms are received.
For freshman year it is vital to get that done early in January, because need-based scholarship money is often handed out on a first come, first served basis. After that, many schools have a later deadline for returning students than for freshman as they figure that if you needed aid your first year, you will probably need it in later years as well.