For a first digital landscape it's pretty neat.

First thing for depth would be a background color. A bright desaturated blue for example. So the bright hills get some more contrast.
And it needs stronger shadows if the sun is in a position like this. Like in the bottom right corner, that looks good already.
Coming back to the blue sky, you can use that blue in small quantities as haze in the riverbed to show the distance.
I always start with a background color that represents the atmosphere.
And you might want to look at reference pictures, googling for "rocky landscape" and hills and such stuff. Carefully observe haze, lighting conditions and texture of the rock. Textures went pertty well on the right side mountains for now, but the left mountain needs work I think.