Successful images give the illusion of three dimensional space on a flat monitor. Creating this illusion of depth involves a variety of techniques, which gives beginners many ways to mess up. Though not a comprehensive list, common methods involve overlapping shapes, atmospheric perspective, and the scale of surface details. This video explores some easy ways to enhance the illusion of depth.
As a final note, all of these techniques are improved with linear perspective. If you don’t feel very comfortable with those skills, I’d encourage you to check out the basics.