Here are some sites to help you get started: DON'T JUST USE THESE!!!!!!!!

http://www.aber.ac.uk/media/Documents/intgenre/intgenre1.html essay about genre theory

http://www.resnet.trinity.edu/wmclean/genre.htm more genre theory and info about film noir

http://www.google.co.uk/search?hl=en&source=hp&q=film+genre+theory&oq=film+genre&aq=4&aqi=g10&aql=&gs_sm=e&gs_upl=1485l5328l0l9594l14l12l2l0l0l0l188l969l4.4l8l0 film genre presentation

http://www.filmsite.org/ great film site overall for

http://www.filmsite.org/genres.html a lot for film genre, subgenres etc....REALLY GOOD!