Shading is the part that makes a drawing go from a flat contour drawing to a 3 dimensional illusion.
There are many options for shading and each option will change the style of your drawing. Here are some of the basic ones, you can try them on a simple sphere design as a fun exercise.
With cross hatching you are drawing parallel lines, varying the direction of the lines and layering them to obtain different intensity of shading.
I like to start by the lightest area and then add layers of cross hatching until I reach the darkest area of the drawing.