Federally and in some cases state subsidized grants (e.g. Pell); for these you must fill out the FAFSA form. Some colleges also require you to fill out the CSS form from the College Board; check each college’s Financial Aid web page for details.
Merit scholarships and special programs within specific universities (e.g. Schreyer Honors Program and Renaissance Scholars at Penn State). Often you don’t even know if you qualify for these programs until you have been admitted; then you receive an additional round of essay questions to complete--sometimes on a very short timeline--to win these highly competitive programs.
And, finally, there are outside scholarships, which are easy to find on scholarship search and matching sites such as College Greenlight. These scholarships are often very specific (based on GPA, for instance, or for immigrants from certain countries, a specific type of artist or athlete, or someone who has excelled in science or community service).