When you see a trill in sheet music how to you know if you alternate the givin note and the half step above it or the whole step above it or any other interval?
I think it's the note that is directly above the starting note in the key. For example if I was in G major and i saw the trill sign on the note B I would play a C because it's the next note in the scale. Or if I was in F♯ minor key and the trill was on the note F♯ I would play the trill with G♯ because that's next in the scale.
However there could be an accidental in that bar and in that case, play that note.
Hope I helped, sorry if this is just nonsense. 😄