FAFSA stands for Free Application for Federal Student Aid, and it's the most common financial aid application. You need to fill it out to get money from the federal government, and many outside scholarships require that you fill one out as well. The form and basic instructions are available online.
The Department of Education, which oversees the FAFSA, also has a website with more detailed instructions on completing the FAFSA.
You can get a copy of the FAFSA at your local library.