The first mistake you can make is not filling out the FAFSA in the first place!
Don't assume that you can't afford college - the FAFSA opens the door to many different opportunities for financial aid, and the sooner you apply, the better, as state grant agencies and scholarship organizations— which often have a limited pot of funds to give out on a first come, first served basis— usually require you to have filed a FAFSA in order to receive aid.
Keep in mind that you don't need to know where you'll be attending school in order to file the FAFSA, and you don't need to have your taxes completed yet for the previous year because starting with the 2017-18 FAFSA, you will use tax information from two years prior (learn more about the Early FAFSA and the move to using prior-prior year tax information). Applying online makes filling out the FAFSA easier because the online form uses skip logic to only ask relevant questions. You will also have the option to retrieve your IRS data to automatically populate the FAFSA, which simplifies the application process, helps reduce errors and lowers your chances of being selected to verify the information on your FAFSA. When filling out the FAFSA, make sure to pay attention to federal, state, and institutional deadlines, take your time to avoid information errors, and not leave too many spaces blank. Be sure to use your legal name, have official documents you need ready, and see what other common mistakes you can avoid.