Based on some searches people have been doing, many people aren't sure whether they are required to fill out the Census.
The answer is yes, every person residing in the United States, even if they are not a citizen and even if they are not here legally, must fill out the Census. The law specifies a fine of $100 for anyone who does not fill out the Census.
The Census Bureau's official website will answer any other questions you have. Among other things, nobody, not even the FBI, CIA, or immigration authorities, may have access to the information from the 2010 Census until 2082. After the 72 year period, the Census makes its information public. This information is, among other things, a valuable resource for people researching their ancestors.
If you want to know what the official Census form looks like, go to this link (also from the Census Bureau). If you get a form asking questions not on the form at the above link, it is most likely a counterfeit Census form, and I would advise you to contact law enforcement to help prevent other people from being scammed.