Hello,
There are resources here:
processing.org
Check out the Simulate examples.
I had a lot of fun dissecting this code and understanding it and now have some cool projects!
Nature of Code also includes particles simulations:
The Nature of Code
This may help:
Processing transition · processing/p5.js Wiki · GitHub
I suggest you glean insight from the examples and work on your own simulation or particle system… when you are ready.
:)