Census in the USA
If you have family from the US, then census records are going to be one of your most important starting points, luckily, there is a wealth of information online. Start with this factfinder from the national archives.
The U.S. has taken a census of its population every ten years since 1790. The census records are available after 72 years, so the 1940 census is the most recent. When it was released, there was a race to transcribe it and get it online.
The questions in U.S. census records varied from year to year and in state censuses, from state to state, you can find information like names of other household members, ages, birthplaces, residence, occupation, immigration and citizenship details, marriage information, military service and more. The US census, in later years anyway, was very comprehensive so it will lead you to lots of other information. Ancestry has a good list of what questions are asked in what census here.
The 1890 census was largely destroyed in a fire, so few records exist.
There are several sites you can use to search the census, usually the 1940 census is free:
Family Search - Free federal census indexes and images to all of the U.S. Census Records except images for the 1880 and 1930 U.S. Census.
Census records.com - You do not have to sign up for life here, you can just pay as you go.
Find my Past - a subscription Internet site, has indexes and images for the US Census 1790 to 1940. 1940 census is available for free.
My Free Census - Find your ancestors in the U.S. census, and International census, free online census records.
Take a look at New Horizons Genealogy to find some interesting census type records, by US State.
They have colonial census records, mortality lists and schedules of pensioners.
The 1940 census is available at National Archives , here you will need to know what state you are looking for. This site also has a lot to offer in terms of information about each years census, so is a good beginning point.
I hope finding your family in the US census leads you to lots more information about them.