Create a scene class that contains lists for your meshes, lights, etc… and also other information such as the scene name, skybox texture, fog configuration and whatever else you might have in your engine. In your main renderer class make a ‘currentScene’ variable that stores a reference to the current scene that is meant to be running. Then in your update and rendering loops you can simply update/render objects from scene that is stored in the ‘currentScene’ variable. For saving and loading look into serialization which allows you to write the state of an object(s) to a file and retrieve it later. Alternatively, you may also want to use your own format for storing scene data in a file.