Click on any note button to play that note. All the green note are in the key of the song (D major), so those should sound good(ish) ;) Also, clicking 1-8 on your keyboard also plays a D major scale, so you can do that too (which is a bit easier :P). So volume dial adjusts the background volume, so you can either hear yourself better, or drown out your mistakes ;)
The backing track is entirely original, and I have been playing around with it on my guitar a bit recently, so I decided to try writing it in note blocks :D Also, the volume control was suggested by @KSeoTCSYL, so credit for that goes to them. Side note: While messing around with this I realized you can play the Quite Uptown song from Hamilton to these chords, so you can sing along to that song too :)