The Federal Census is an easy way to start looking for your ancestors. The 1940 census will be available in April of 2012 making the start a little easier for this generation. It should help greatly for finding post WWI and pre WWII individuals.
Currently the only starting point is the 1930 census and you will need to know the NAME, BIRTH YEAR, Approximate PLACE of RESIDENCE of the person you are looking to find. It does help greatly if you know who they were living with and other details about thier lives. Someone alive in 1930 would be 82 in 2012 so for some of you that would be your grandparents or great grandparents.
There are many pay sites and many ways to search the census. I dont advocate for any particular site and hesitate to even mention the one I use but this site is a good site to start with – Family Search and its free!