Here's a rule of thumb. Which ever side of the body or object you start placing the highlights on first (left side, right side, underside, upper side) that's where your light direction is going to be coming in from.

So make sure you keep the highlights(or if casting shadows do the opposite side [EX: Light hits left side, shadow is casted on the right]) that side of the body and limbs, part of the object, etc. as you apply them.

I don't know of any tutorials to link you to off the top of my head but a quick search of Google pools up a couple.

I know Deviant Art and PIXIV should have plenty as well.

And then my advice is also subject to change when you want to deal with multiple light sources and how they cast a shadow/ where it casts.