The National Archives (NARA) website can be daunting to the old hands, not to mention how it can over-whelm the first-time visitor. But you can take a shortcut and go right to a "getting started" page that can be the home base during your NARA website explorations.
From this page you can choose links to several groups of most frequently accessed records to include Census, Military, Immigration, Naturalization and Land Records.
The NARA web site has very few actual records online. Primarily site provides research tools, such as microfilm indexes and finding aids, articles and information on where to find the records and how to access them. However, there are a few exceptions. This getting-started page gives you a list of those groups of records of genealogical interest with online images.
Check all this out at: http://archives.gov/genealogy/start-research/