Hi Mussa,
Sorry for the delayed reply.
For information on getting started, see this post:
As I said in that post, you're unlikely to find help about specific domains or designing specific simulations in this forum. It's impossible to point you at specific file(s) to start with. Every simulation is different, and I have no idea of what your sim looks like or what it does. My advice would be to identify an existing simulation that has some feature that you want in your simulation, then copy/adapt what is done in that simulation.
If you have specific features that you want to implement, or questions about how to use specific PhET APIs, that's where this forum can be helpful. Be sure to provide details about what you're trying to do -- screenshots, UI designs, etc. The more details that you provide, the more likely that members of this forum can help you.
Chris Malley
PixelZoom, Inc.