I assume you have a pen tablet and if not then go get one, if you seriously want to do digital drawing.
As for line work, I'm by no means very good at it, but what I've come to realize is doing perfect line is not always the best option. Firstly you need to use pen pressure to vary the line weight in different areas. Also use different brush sizes. Example outline 10px, details 5px and overlapping stuff 7-8px.
Secondly, don't use perfect straight lines when drawing something organic, like the middle line on the turtle shell.
Thirdly, if you use some scribble here and there, messy lines indicating nooks and grannies on the figure. Muscle lines, skin folds etc. It really helps to bring the drawing alive. Also for the turtle skin texture would help a lot and more highlights, since it's skin should be a bit slimy. That's what I have to say about line art.
You have the overall shading idea. But I would use darker shadows and/or lighter highlights, to bring out the shapes more. Right now the turtle, even though its shaded, it looks like its flat. Also he's looks too skinny, should have more muscles, look at reference images of comic books.
And lastly what I would suggest. Do not try to draw from imagination. A lot of people think using reference is "cheating", I think its a stupid way of thinking, everyone use them, even pro's who have been in the business for decades.
If you have a certain idea what to draw, then look up references from google. For example, if you want to draw a ninja turtle, look at comic books, movie screenshots, cartoon screenshots, etc, mix in some of your own ideas. Turtle wielding a taco gun or whatever. More you use references more you improve your visual library, linework, composition etc. REFERENCE IMAGES!
Good luck!