Hi Bennie, welcome to the Group!
If you haven’t already, start by consulting the resources that I mentioned in this post:
After that... Yes, figuring out how to get started with creating a new sim is a daunting task. It would be great if a tutorial or user manual existed, but PhET currently does not have those resources. Your best options are:
(1) Study example-sim.
... with these goals:
* understand how the MVC (Model View Controller) design pattern is used
* understand Property, and how you can "link" to a Property to be notified of changes
* try to add Nodes, understand how the scenery scene graph works
(2) Copy/adapt existing code.
Find a simulation that is similar to your simulation in some way. For example, if your simulation needs a ruler, then look at beers-law-lab, which also has a ruler. Ask in this Group if you need help finding a simulation that has a specific feature. Then copy/adapt the code to your simulation.
(3) Ask for help.
If you are stuck on a specific problem, ask for help in this Group. Describe what you’re trying to do, and describe the specific problems that you’re encountering. The more specific your problem is, and the more details you provide, the more likely you are to get help. For example, you said “Im already having trouble setting a background image.” That’s the kind of problem that this Group could help with, but not without more information.
I hope this helps.