Shading Tips???

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So i suck suck suck at when it comes to shading my drawings cause i never learned about light source cause i never really paid attention in art class, i mean I was half asleep during most of my teachers lessons cause he's always mono-toned and i don't think any of ya'll would be awake during anything like that...anyway i just need advice for any talented artists out there PLEAS I NEED HELP!!