Samefagging to add, shadow is mostly just a converse of light. If the light is from above the lower edges need shadow. The basics of directionally consistent relatively high contrast shadow will make your art already more notable than most.
The midtones are a bit more difficult and it's highly dependent on where the character is. Indoors the contrast grows with shadows being darker but if there's natural light (windows and doorways and stuff) you might find worth in a grey tone further from the light than the high contrast black.
Honestly a great resource is looking at artists who do inktober. Samnee usually goes slide by slide and includes greytone steps.