Welcome to pixels and art.
I'll link to some pretty old tutorials:
lighting:
details:
To factor in a perspective difference, it's difficult to explain but think of it as a cube. Start chiseling off parts into a rocky shape, rotate it towards you (or just roll it 90deg), bam, you get a rock.
Or here's another tutorial showing basically the same thing.
Notice the first two start by painting blobs instead of drawing outlines. (pixel-specific, important to this workflow is clusters, but it's heavy stuff to get into for beginners. I'll keep it simple.)
I would advise you to do this, don't even look at the 1px brush until you have taken 4px and then 2px as far as you can.
General > specific. Good steps to take unless you seriously seriously know what you're doing.