Assuming that you're interested in developing sims using the PhET libraries...
* Study PhET's example-sim repository,
https://github.com/phetsims/example-sim. This example is typical of the structure of most PhET simulations. Experiment with this example, attempt to make changes, learn how it works, use it as a starting point for your own simulation.
* Ask specific development questions in this Google Group.
Chris Malley
PixelZoom, Inc.