Contouring is used to emphasize bone structure. You can use a lot of different products, from a slightly darker than your skintone foundation or concealor, to a bronzer that's not too orange or too shimmery, or a specially made contouring powder product, and highlighters that are either powder or liquid.
Contouring can be using a darker shade to emphasize hollows or shadowed areas of the face, highlighters to bring out the higher planes of the face, or both.
With a darker shade, you blend into the hollows of your cheekbones - feel where your jawbone is starting from next to your ear, and shade from there to about where a dimple would be on the cheek (whether you have one or not!). You can also do a little dark contouring down the sides of the nose, on the temples, and under the jawline. Blend WELL! There should be no lines, it should be subtle. Try doing it on just one side of your face first to see what a subtle difference just a little can make. I just use bronzer.
Then, if you want to emphasize even more, use some highlighter. This is going to be a shade lighter than your natural skintone, usually slightly shimmery or pearlescent - but don't go sparkly or TOO shimmery! It should make you glow! Apply this to the top of your cheekbones (in a C shape starting from just under the ends of your eyebrows, down and around to under the outside of your eye - follow the orbital bone). You can also do a little bit down the bridge of your nose, and dab some in your cupid's bow above your lips!
Hope this all helped! The easiest way to figure out where to contour is to just feel where the bones in your face are!