I know you've said you didn't want help with the outline but with shading. But in order for it to be convincing you have to start off in the correct way.
I always say the best way to illustrate form (which is what shading is) is to understand the structure from the inside out.
To become adept at drawing figures you need to begin underneath and work out from there... i.e. skeleton > muscles > clothing.
I'm not suggesting you need to have a doctorate in anatomy, just an understanding, the more you do the better you get.
What I see here is obviously a trace from some other source which is fine and fair as a beginning way to create pictures but you won't learn fully how to construct form using this method and ultimately it will restrict and limit what you can create by copying from photo's or other peoples work.
This line art you have here is SURFACE form, what I'm seeing is clothing, there's an underlying structure for sure, but in order to push this on and create the shading you require you need to SEE the underlying form, more importantly understand it and how it relates to the way cloth hangs and so on on top of it.
The classic mistake in other words.