Strumming chords and singing is fun but I find that I have the most fun by playing the ukulele as a pianist or an organist would – by playing the notes of the melodies along with the chords. It isn’t near as hard as it sounds. It starts with learning the notes of the fingerboard., This video created by Phil Doleman is great for learning those notes! Give it a try!
If you play a string instrument and want to learn the notes on each strings, the notes on music staff, etc. the FretTrainer APP is good.
There are 6 instruments available to practice on: Guitar, Bass, 5-String Bass, Mandolin, Ukulele, Banjo.
It is free, but has ads.
This is an excellent and short tutorial on how to read music. It is only 5 minutes long and offers a good foundation to get started reading music. It is short enough that you can (and should) watch it several times until you understand it.