Often you will have 3D polygonal data for an object, but need to generate normal vectors for it, in order to light it.
To compute normals from polygonal vertex data, use the cross product.
The cross product produces a new vector that is perpendicular to the two multiplied vectors. If the two vectors are edges of a polygon, then the cross product will give you a normal vector for the polygon (you will need to normalize its length to be 1.0, however).