You'll be able to use the 1940 U.S. Census online at no cost starting at 9 a.m. EDT on Monday, April 2, 2012, as long as you have Internet access. [If you don't have the Internet at home, you should be able to use it free at your public library.] That's just a week and a day from now.
The National Archives will make that digital scans of the census pages available online here:
Ancestry.com will get its copy at midnight on April 2 and will begin working around the clock to get it online and an index created. Its copies will be free to use through the end of 2013:
And FamilySearch will use a vast team of volunteers to build its index to the 1940 Census and that will always be free to use. They expect this volunteer effort will take until the end of 2012, depending on how many volunteers help the effort: http://www.lds.org/church/events/familysearch-to-launch-major-1940-...
Old Fashioned Genealogy at First
In the beginning, there will be NO INDEX to the 1940 Census, so it will be finding family the old fashioned way, browsing page by page looking for them.
To help you find family members more quickly, there are online tools to find the Enumeration District where they are listed. Go here to learn more about EDs:
The 1940 Census has a remarkable wealth of family information for you to find and use. It shows where a person is in 1940 but also was in 1935. It is a very detailed census coming at the end of the Great Depression and before World War II for the USA. See the census form here:
Best wishes in Finding Family for Free in the 1940 U.S. Census!