One of the most important thing is to look at the details in the real world and work on it. More you work on the details, more the scene will be realistic.
To set lighting in a scene, you need to know some photography basics to understand camera settings and white balance.
Start with a small scene and shoot it like a photographer, remember the composition - it considerably changes the image perception.
I think the most difficult was to start… There are so many things to learn about 3D that when I started I did not know what should I learn first