welcome to the world of digital, my dude! c:
It's hard to get used to the hand-eye coordination, but just keep finishing pieces from start to finish on it and you'll get it eventually!
For this piece, I think the first thing I noticed is you appear to have clashing perspectives. for the city you should have been referencing two vanishing points. These two vanishing points would be placed on the horizon of the piece with one towards the right and one towards the left. These vanishing points should be NO LESS than a 90 degree angle apart from the viewer of this picture (not the woman, me, viewing this piece).
Moving onto this woman You really gotta study humans to get them down. They are complex as fuck and I study them everyday and still fail to impress myself with my work.
Things to look into:
3D SHAPES
PERPECTIVE
CONSTRUCTION OF OBJECTS USING 3D SHAPES
PROPORTIONS
GESTURE
I'm drunk atm, if you have any other questions, feel free to DM me.