Well first of all you need to keep in your mind at least one spot of light. After that, you just imagine how that light should affect the colors of the object you´re colouring, like "this part is in front of the light, so its back will have a shade". That´s the pretty basic, but the way you´ll select/put the colors will depend on the shading style you´re using. Try looking at the picture as a whole and imagine all the shades, and after taking a brief idea, go zooming some parts of the drawing and shading them separatedly.
Like, for cell shading you first select one basic color for all elements of the drawing (skins, clothes, trees, or whatever). After, for the shades, you select that basic color, darken it a bit (or lighten, if you want to highlight something) and brush in the drawing, where you think the shade would be. I just advice for you to put the basic colors and the shaded colors in separate layers, so it will be easy to correct some mistakes or even add some effects. Cell shading is very easy and straightforward, and there´re tons of tutorials on it. If it´s your first time colouring a drawing, try shading in that style for a while, until you feel confident enough to jump to a better style ^^
I´m trying soft cell-shading now, it´s really wonderful, and as easy to use as cell-shading, but with a bit more of work. I generally use one layer for the lineart, one for the basic colors, and masked layers for shadows (2 or 3 layers) and highlights (2 layers). There´s also those layers specifically for some parts, like effects, backgrounds, and the like.