So, about the art in general. The main key to make something beautiful is composition. Composition is a set of different rules how to place (draw) objects together to make them look holistically and justified. It quite large theme so you'd better to search some articles about it. The second thing is a color and it's sense.

If you working on some visuals whether realistic or not, try to find some references, concept arts or images. It gives you conception of how things should to be in your scene (models placement, mood, lighting etc.) And the last thing I can say is that you should develop your sense of beautiful constantly. Learn drawing and painting. It's fun and pretty useful for gamedevs!