How do i do proper shading and lighting for this?

[crude pebble]

I'm thinking of doing low poly graphics shading but don't know how mix it well to get that 3D look (i use Sketchbook)

[proper lance]

why dont you create a copy with the character silhouette in a neutral grey? you can then shade the basic lighting without worrying about the fine details. if you need additional explanation, im happy to demo with a video or an example