Music is a very abstract thing, but there are some foundation guidelines most songs follow called “theory.”
Music theory isn’t necessary to learning the ukulele – many great players have done without – but it will explain a lot and make you a more rounded musician. It isn’t meant to replace what you already know, just to clarify the origins of notes, scales, chords, and keys.
It’s very easy for “Western thinking” people to get caught up in trying to riddle a song out like a math problem. Don’t fall into this trap. Music is art. If you try to put it in a box it’s going to make your life difficult.
Theory follows a progression and should probably be learned in order.
The notes of Western music are a good place to start.
Once you know that B and C are neighbors along with E and F, you can start:
From a scale you can create a chord.
Diatonic Harmony (aka Chord Scales)
If you think of triad chords as three offset scales, you can figure out exactly what chords naturally occur in any key:
Finally, you can move the pitch of a song up or down to fit the range of the instrument or your voice. This is called: