The confusion between compositing and composition seems to still not be clear but I also will disagree with everyone who says that composting should even be a thought in your head at this point. Your lighting needs a lot of work.
People here are being very kind in their critiques, but I think it’s doing a lot of disservice since you clearly want to learn how to approach lighting.
First, lets get the composition out of the way so we can focus on lighting. Leave the scene with the last composition you have. While the compositon could certainly improve a little, I am assuming you want to focus on learning to light, and in lighting, while scenes may be well composed initially, camera and characters move around so much that not every frame will be a good composition, so we can think of this as just a random frame in a scene. You could always fix the composition of your scene once lit, it may require a tweak or two to you lighting but nothing major.
Vid2K had the best suggestion so far which was to throw out your lighting and start with a simple three point light setup. I absolutely agree that you could be stuck for ages trying to tweak what you got, not to suggest that it is so bad nothing can be done to fix it, that isn’t what we are saying, but often especially if there are too many lights or too many things going on, it can be harder to fix than to just start over with something simple. Really I’d actually disagree for this specific situation, with Vid2K in that I wouldn’t even start with a 3 point lighting setup. too complex for a scene that won’t really work well with that type of setup. Studios, single objects, those will benefit form a 3 point lighting setup right off the bat, but here I’d start with on light. so literally get rid of everything and get your first light set up.
This first light will be your key light to the scene. Where do you want this light to be? well that really depends on two things, the first being, is there a definitive source of light in the scene, like the moon or sun typically. If yes, than this will most often be your key light. The other factor to consider, that may sometimes override the idea of using a sun or moon as your key even if they are in the shot, is the mood. Perhaps you want something where the sun or moon should only be an accent or rim because your trying to create a very specific mood, so something like a fire or a bounce of light is the more important light to establish your scene. I’d say as a beginner it usually will be better to focus on the strongest most prominant light being your key. By the looks of your scene it has been a sun, but in your last image it looks like you may have a moon in the BG? Figure out what time of day you want this to be as that will decide the key light’s color and intentisy. If it is a sun it will be warm in color and quite bright. If it is a moon it will likely be cool in color and dimmer, probably with a slightly softer shadow Dusk or dawn and your going to be pushing even warmer with the sun into reddish orange.
Get a render of that and post it here and we can critique that, figure out what is working and what is not before we go onto any other lights. It will mean you have a somewhat dark render with areas that are in total darkness, mainly shadows. It should however mean that we can see most the scene and that there is some shape. If you first light isn’t getting a clear shape, then we’ll know there is something wrong with your light or your shaders.