clicking the right mouse button and add an object type sims 2 or 3 that system to add objects in the house and in town please i await reply.
Your question is incredibly vague. Based off of what I know from the Sims, you want to be able to right click to get a context-menu of sorts, and then a left click on that context menu will create an item?
Or you want to have a catalog in the pause menu (the last Sims I played), click on that and then click again to place into the world?
Either way, you’re going to need to research the following:
Input (for detecting mouse clicks)
GUI (for drawing context menu or pause menu)
Object modeling (Blender is free)
Prefabs
Object instantiation (see Instantiate command)
These few things are the very least of what you’ll need to do. You’ll likely need active collider detection for object placement.