Not sure if you posted this in the right section, but w/e.
It's pretty easy, just copy the title of the movie you've download, go to google, paste, add "subs" or "subtitles" and search. Most likely this first hit will be okay.
Download the subtitle, which is a .srt file most of the time (sometimes a .sub)
Make sure the name of your movie has the exact same name as your subtitle file.
Example movie: AwesomeMovie.mkv, subs: AwesomeMovie.srt
You also need to put your subtitles in the same directory as your movie.
Now, I don't know if you're going to connect your tv to your pc, or going to play it on your pc from a usb device.
In the first case, download VLC (VideoLAN) and use that to play your movie.
In the second case, just put both movie and subs on a usb storage device (make sure they're in the same directory), plug it in to your tv, and play.