Go to InternetMovieDatabase.com and enter any information known on the movie into the search field. The best way to start is with a character or actor's name.
Scan the search results to see if any of the information listed triggers your memory on the title. If not, click on one of the links for more information. Hopefully, the title character or actor's name will appear. If so, click on their name.
Scroll through the Filmography until the title appears. If the title does not seem to appear, begin to click through the actor's Filmography titles to read the plot summaries. This should lead you to your movie.