The first step, and often the only one, is to go to one of the websites I have listed in the resources section. You will input the title of the film in the search engine, and selections of subtitles will appear. You may have to scroll down to find the English subtitles, as they will usually have them in multiple languages.
If you can't find the English subtitles you're looking for on one of these sites, google it! Put the title of the film in quotation marks (") and in the original language, and then put "English subtitles," also in quotation marks. One of these other sites may have what you are looking for.
Sometimes you will have many different sets of English subtitles to choose from. So how do you know which is the best translation? It's a little difficult. Usually, it's best to choose a few of the highest rated ones. If you know some of the film's language, put on the subtitles and go to a certain scene where you can see how well the translation is.