For articles, see the Academic Search Premier database. For encylopedia articles use Credo Reference Online. American Cinema is a vast subject, so you may wish to limit your search in some way: 'American AND cinema AND westerns', for instance. Be sure to use movie, movies, or film in your seach term, if you are not getting good search results. For books, search the the library catalog in similar fashion.
Look for the quick links I've provide to these resources in the box below this answer. I'm including a link to the Finding Articles tutorial, you may want to view.
Stop the Reference Desk or contact a librarian, if you have further questions.