Welcome to Inkscape Community!
If you click on the Home tab near the top of the page, you will find more tutorials than you could hope for. Around the middle of the page is a section for Cartoons/Comics/Game Art. Any of those will help. But specifically, this one may be the best:
http://2dgameartforprogrammers.blogspot.com/ Note that each new tutorial on that website, builds on lessons learned from previous tutorials. So it's meant to be used from beginning to end (which is how I have them listed - click on the black text and it will open the list of tutorials)(you'll see the finger when it's time to click).
You need the Paint Bucket tool
to fill areas that are not closed paths. The area has to be closed, but it can be made of more than 1 path. There are several ways to create shading, and you'll just have to practice to find the way that works best for you. Some people might use Gradients
. Some people might use bluring, which is not a tool but simple blurs are in the Fill and Stroke dialog. There are much more complex types of blur in Filters menu. Some filters alone can make an object look 3d. But comics usually aren't like that.
And, plenty of people use flat colors for shading. You just have to draw a special path for the shading. Chris Hildebrand's tutorials will show you how to do that.
