In general, you should complete the FAFSA as close to the beginning of the year as possible, for every year in which you plan to attend college that fall. The FAFSA can be submitted on the web from the beginning of January through the end of June, but the earlier you submit it, the better your chances of finding financial aid. Depending on your age and other factors, you'll need to know either your or your parent's tax information; the government uses this to determine your expected family contribution and thus, how much aid you should need to attend school. Information you'll need to complete the form includes the number of people in your family and how many of them are attending college, your (or more likely, your parents') income, and non-retirement assets.
A new FAFSA should be filed every year, but the system will remember your information from one year to the next. If you file your FAFSA online, not only will it be much faster than sending in a paper form, but most of the information will be prefilled - just don't lose your PIN! You'll just need to note any information that has changed, as well as filling out the current year's tax information.
After your FAFSA is processed, you'll receive a SAR - Student Aid Report. Check it over carefully for errors; you can make corrections online using your PIN. You can also view and print your SAR from the FAFSA website once it is available; the government recommends checking the status of your application one week after submission if you signed it with a PIN or 2-3 weeks after submission if you mailed in a signature page.