When you place objects, you need to check your objects to see what they are set as. With your placed object selected, go to the General tab under Object Tools. See what it's set as under Static Mode. Is it Static or Physics On/Off? If your player is not going to interact with the object, then set it to Static.
Next, make sure you have access to Developer Settings. If you don't see that panel under Object Tools, go to Edit>Settings, go to the Developer tab, and check Display Object Tools Developer Mode. With that checked, you can now twirl down Developer Settings (again, under Object Tools on the right-side) and look for IsImmoble. Check that. This will make sure that any physics object will stay still.
Posters, signs ... should all be static objects. By "lights", I take it you mean light fixtures (not point or spot lights that actually create light in the scene)? If so, they should be static objects, too, with IsImoble checked as well.
You might also want to check your table. It should be a static object with box collision. It could have polygon collision, too, but that's pretty intensive for this table. Box is better. In other words, if that table doesn't have any collision associated with it for some reason, the weapons, etc., will just fall to the floor like nothing is there.
Intel i9-10900K 5.10GHz, 64 GB RAM, Nvidia GTX 1080 ti 11GB, Windows 10 64-bit, dual monitor display