Perhaps the best thing to do is to review a blank FAFSA form before you actually sit down to complete one online. If there are categories or terms you don’t understand, you can contact your tax preparer or seek advice elsewhere. Again, you should avoid waiting until right before the deadline, since it may take a while to get answers to your questions.
As mentioned, accurately completing your FAFSA is going to generally require you to have your most recent year’s tax return completed. Not having this will leave you guessing on a lot of numbers and will require you to go back and correct the information later.