Contemporary reviews of films are often important for scholars of film history. They can be quite easy to find for films released recently, but can be difficult to dig up for films released before the 1980s. There's also a difference between the popular critics, such as those who write for Entertainment Weekly or the newspapers, and the more scholarly critics, who write reviews for film journals from a vastly different perspective. These resources will help you to find movie reviews of all different types.
Citing your sources properly -- especially the films that you refer to and any images you include in your paper -- is important and required.
Purdue University's OWL Writing Lab Citation resources are the best ones out there on the web. Generally, Film Studies courses require that you use MLA style. Detailed information on MLA and other styles, such as Chicago, are available here.
Check out Berkeley's Guide to Citing Film, Video, and Online Media.
Diane can help with citations, too!