It's easier than you might think to learn how to go about matching the melody notes (the tune) of any song to the appropriate chords. It's really a simple 2-step process, once you understand it, but if you don't know the secret, you can wonder about it for years. But if you learn it now, you will know it the rest of your life.

Back when I was a teenager learning to play the piano, it used to puzzle me which chords to use with which melody notes:

"I wonder how those top pianists know which chords to use? I mean, they don't always use the C chord to harmonize the C melody note, or the D chord to harmonize a D in the melody, and so on -- so how do they do it?"

A few years later I had the extremely good fortune to find a teacher in Hollywood by the name of Dave. He was THE teacher -- he taught many of the recording artists and names you would recognize -- plus a young aspiring piano player named Duane (me). And he showed me how to go about matching the melody notes of any song to the appropriate chords.

That changed my musical life. I didn't have to wonder any more -- I knew for sure which chords went with which notes, and when and why.

It's really a simple process once you understand it, and it has to do with knowing just 2 facts:

Fact 1: There are 3 basic chords that will harmonize any note, and that note is a member of all 3 chords. For example, in the key of C there are 3 basic chords (in music theory they are known as "primary chords") that are organic to that key because they are the only 3 chords that occur as major chords without having to add any accidentals. Those chords are the C chord, the F chord, and the G chord -- also known in music theory as the I chord, the IV chord, and the V chord (Roman numerals are used in classical music notation). So if you play any note of the C scale -- C, D, E, F, G, A, B, C -- you can harmonize that
note with one of the 3 primary chords -- either C, F, or G -- because all of the notes in the C scale are members of one or more of those 3 chords.

Fact 2: These 3 chords rotate as the melody moves through the song, so pick the chord that has that melody note in it. For example, let's say you are trying to pick out "Silent Night"
by ear, and you start on G. Simply ask yourself "Which of the 3 primary chords -- C, F, or G -- has the G note in it? The answer is both the C chord and the G chord. So you try one, and
if it doesn't sound quite right, try the other. Before long you will be sensing which chord
is the better choice. Like in any subject, there is a learning curve, but it's not a steep learning
curve -- especially when you enjoy what you're doing!

Then once you master the primary chords, you can widen your scope to "cousin chords"
(known in music theory as "secondary chords") then later on you can add "neighbor chords" and "color chords" and all kinds of other exciting variations, and then the sky is the limit in terms of improvising and making up your own musical sounds.