Hi, welcome.
Where are you stuck, and what have you already tried?
Lighting is a common thing to do in 3D engines, have you tried searching the G3D thread to see if others have some previous work that you can start from?
For example, searching for the word "lighting" only within that thread (the search bar below the thread title):
search.php?keywords=lighting&t=86350&sf=msgonly
Toon lighting is standard lighting that's been quantized. Like quantizing it to two steps so you only have lit / unlit colors that cause a very sharp shading, or using more steps for a softer transition, or even a custom gradient ramp to fine-tune the look.
Toon lighting effects are in a category called NPR (non-photorealistic rendering).
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