So create the ReflectionProbe node. Set the extents (the box size) to the general size of the room. It works best on rectangular sized rooms. Set Interior to true if it is inside a building or room. If it is outdoors and you want the sky reflected, then leave Interior off. On your material, make sure it is reflective. You can do this by setting Roughness to a low value like 0.2. You can also make Metallic higher like 0.8. This should be all you need.