Ok so I'm not super familiar with "Say Something" by A Great Big World ft. Christina... But when I'm trying to learn how to play a new song I like on the piano I usually watch someone else playing it a few times and then try to get into the flow of the song.
After that, all you gotta do is "practice, practice, practice!!!' as they say.
Anyway, if there's noone around that can guide you through that song, the obvious help you can get online is from YouTube.
I looked up the song for you, hope its the right one you mean.. And there were a few versions that are pretty easy to follow along to (and I am still at beginner level)
I included ones that are slowed down, even though its a pretty slow song anyway, so you should be able to play along nice and slow at first.