okay, typically how i'll do it is have one layer for the base color, another for simple color blocking to establish light & dark, and another for the actual shading which is where the fancy color mixing and whatnot goes down
shadows and light are way way more than just lighter or darker versions of your base color mix in stuff like blues purples, whatever into your highlights and shades. individual hues vary a lot depending on a lot of things - light, environment, skin color, etc. when people shade with just lighter and darker variations it usually ends up sort of bland and unappealing looking, or in the worst case, like a dodge & burned deviantart disasterpiece
defining the texture and intensity of the light is really dependent on contrast - things that reflect a lot of light, like metal, are very high-contrast as opposed to things like clay or mud
don't expect do be doing a lot of complex shading right at first. there's nothing wrong w/ getting comfortable with simple light sources and basic shading before you start trying to really push your colors and other things
just look at things - look at your hand, your desk, your room, everything as you're doing it. really LOOK at stuff and the shadows with a learning artist's eye
hope this helps, i'll try to dig up some less rambley & more helpful tutorials later