Here is a somewhat simple song to play on piano I've tried to learn yesterday.
It might not be suitable for absolute beginners, and you need a sustain pedal – tho you can perhaps emulate that by drenching your digital piano in reverb – but it's a good left-hand exercise.
You don't have to play all the notes of the left-hand chords at the beginning, you can just play the root notes of the chords instead.
Here is a tutorial for the song using Synthesia software: https://youtube.com/watch?v=OTnhlQq_Vi8>>4766
I know, and to be fair, if you are serious about making music with a computer, you should rather get an audio interface because most of them have MIDI ports, even if it's just a Behringer U-Phoria UMC204 or a second hand Focusrite 2i4.
The Casio keyboard OP got doesn't have a MIDI out DIN port as far as I know, only USB.