The United States Constitution requires the government to take a complete census of the population Every ten years. At different times, different material has been gathered. Material is sealed for a number of years--the 1930 census material has just become available.
Figuring out census records is one of the most confusing parts of genealogical research. Most material is microfilmed, and local libraries often only keep those microfilms related to their area. There are several places in the nation where a complete set of the records that survive (most of the 1890 census was destroyed in a fire in 1921)--the National Archives in Washington, D.C. (which has a branch in San Bruno, CA, a ten minute drive from San Francisco) and the Archives of the Latter Day Saints (Mormons) in Utah are the foremost depositories. Furthermore, the Mormon Church has local reading rooms throughout the country which can get access to particular records.
If you want to do serious census research, you need to read up on it. The best place to get started for free is the U.S. GenWeb site at Rootsweb.com. "http://www.rootsweb.com/~usgenweb/" I haven't included a link because clicking on that link will take you from this site in a way that is not easy to backtrack.
You can't just decide to find someone named "Kellogg" in Red River Parish in 1920, for instance. You have to also have access to an index of names for the period, and a lot of perseverance. The reason is that a lot of mistakes have been made in transcribing records, so "Totsy Franklin Kellogg" might show up as "T.T. Kellogg", with an "F" confused with a "T".
Increasingly, the best and easiest source for census records is Ancestry.com It's not cheap--an annual subscription can run to $200 or so, but there are frequent sales, and quarterly membership rates. But with your membership, you can access most of the actual census images on-line and save them. That's the source for many of the census images I've included here.
I have included some of the documents listed below, but space limitations for free web pages has meant I can't make them all directly available to everyone, but must instead give you a basic description of them. If you need a copy of the census materials, please write to me direct at keller.bird@comcast.net