Many tools, databases, and repositories are available for family history research, and for historical context research. In most cases, U.S. Census data is the best place to start, both for information about your ancestors, and also for contextual information about who lived around them. On census records, you can find out not only the names and ages of people in households, but also other information such as occupation, ability to read and write, whether the home is owned or rented, and property value – depending on which questions are asked in that census.
You can access some census records for free by going to the history section of the U.S. Census website. From there, you can go to the National Archives, which houses U.S. Census records. There, you can either search the 1940 census for individuals, or browse locations (such as addresses or enumeration districts). Links will also take you to additional useful information such as questions that were asked in a given census, and even links to immigration records. American Indians were not included systematically in the census before 1900; links will take you to census information about American Indians before then.
You can also access census records (as well as other genealogical records) for free at a local family history center. Family history centers, provided by the Church of the Latter Day Saints locations, can be found in most towns. I’ve even found centers in small villages, sometimes in a corner of the public library.
If you don’t mind paying an annual fee, you can access a wide range of census records, along with numerous other kinds of records, in online databases. Ancestry.com is the most well-known, and provides a “one stop shop” for a good deal of information, plus tools not readily available elsewhere. As the largest for-profit genealogy company in the world, you need to pay to join. Family Search, which is run by the Mormon Church, is free, and will give you information available on census data, although to see copies of documents you may need to register with Ancestry.com (the two are now working together).
U.S. Census records have limitations and inaccuracies to be aware of. It is difficult to track women who change their names after marriage; I have lost the trail of many daughters for this reason. Birth dates are readily available, but of course death dates are not in the census and need to be found elsewhere. Some records have been lost or destroyed; for example, only fragments of the 1890 U.S. Census exist.
Names are often spelled as the census taker heard them, leading to varied and sometimes creative spellings. For example, in one record, the name Augustine as spoken by recent immigrants was heard and recorded as Ogletree. Sometimes people lie or reinvent themselves by shaving years off their ages, changing birth locations, even racial identification. Birth dates depend on memory, which is not necessarily accurate. Census-takers are given directions that vary from decade to decade, such as directions for how to classify people by race. Be creative when looking for information, and careful when interpreting it.
One more caution: locate as much corroboration as possible for anything you think is true. In census data, it is easy to identify people as ancestors on the basis of their surname, but sometimes these are not actually your ancestor. The growing number of family trees posted on the Internet can add to misidentifications. I have found inaccuracies in one family tree circulated and recirculated in other family trees. As much as possible, triangulate findings -- seek names, dates, locations, and family connections from multiple sources in order to be sure that the people and relationships that seem accurate hold up under scrutiny.