Subs.to is a simple subtitle search engine. Plug in the name of the movie or television show—make sure to include the season and episode number!—and the language you’d like to read the subtitles in and Subs.to will return the subtitles in SRT or SUB format.
For most media players all you need to do is place the subtitle file in the same folder as the actual movie file—i.e. movie123.avi and movie123.srt—and the player will use the file without any interaction from you. A quick Google query of “yourmediaplayer subtitles” should clear up any issues you run into. [Lifehacker Annotation]