Shadowing question

how can i get images that are photoshopped to look real? is there any specific way to do the lighting and shadowing?
how can i get these attachments to look real and part of the background? is there any way to do that?shadowing 1 shadowing 2

Shadows would definitely do the job!

Yes, you need to add shadows and reflections. For the first one, you would add subtle shadows on the bottom of the cylinders and underneath on the table. Use soft edged, low opacity burn tool bigger than you think you need to get the most control and most blended look without the shadows looking painted on. For the other one, seems like you would need reflections in the table. The current white/black area on the left looks a bit unnatural, you would just paint or clone stamp the color straight underneath to create a reflection effect. Or copy the item and position it underneath, then play around with opacity and erasing part of the reflection as it goes down so it is only strong right under the item. A little darkening on the bottom of the items would also help.