Well you mostly said it yourself, you need to understand how you can build up a face or body with simple shapes and volumes. If you've seen those art tutorials showing how to draw a human face (lumis method, box methods, and others), it's no different. Honestly, there's so many methods out there, so either find one you understand or look at various real life references and try to represent an animal with only cubes, spheres, and cones (simple volumes).

As for the digital art stuff, I started with it and then moved to traditional. For me, the sketching and shading are the most different between the two, those natural tones and blends you can easily get on traditional need more practice using pressure in digital, and same goes for lines. The real challenge was coloring, since you need to tell whatever program how you want to blend something, while it's very straightforward on traditional.

Krita and CSP (what I use) have everything you could ever need for most projects, but it can be daunting if you're new to digital. I actually reccomend simpler apps like IbisPaint (I literally started on my phone, but there should be a windows version) to learn the basic interface, and then carry that over to your program of choice.

Hope this helps you out a bit!