I don't think a tutorial is really needed for this, it's a pretty simple concept. Just experiment a bit with whatever program you're using, I'm sure you'll get it in no time.
Each normal is tied to a vertex, and the orientation determines the direction in wich light is reflected to. In general, for correct lighting you'd want to keep the normals perpendicular to the surface (picture below, right cube). Note that this causes edges to look quite sharp, wich is very fine for buildings, crates, metallic units, etc.
When you're trying to model something with soft edges or even curved surfaces, like a limb, a tree, an arc, a sphere, then it's different. You still want to keep the normals sort of perpendicular to the surface, but you need to get rid of the sharp edges effect, wich is caused by normals facing many different directions. This is done either by welding all coincident vertices (but may mess some things up, like texture wrap), or snapping their normals so that they all points to the same direction (wich should be the average of all previous directions, see the left cube).
Now if only I had a 12-sided cilinder primitive I could show it better, but I'm sure you got the picture.
