As with most areas of art, there are uncountable ways to apply shading to a drawing. Here is a simple way if you are having trouble getting started...
Draw out a value chart. Value is the term used for lights and darks...high value is light, low value is dark. Having a chart to refer to can really help you analyse how dark or light an area of your drawing needs to be.
In almost every lesson I find myself saying 'make your darks darker and your lights lighter' - it's become a running joke with my students! This is because the work can often look bland if it is limited to the mid tones.
When you have completed your line drawing you need to look, look and look again at the very subtlest light changes, all these details make a massive difference. (Take a photo and convert it into black and white if you want help, it’s worth doing this to see the difference in value).
Then you need to apply hatching and cross-hatching – diagonally in both directions, vertically and horizontally.
‘Ring-fence’ areas of the drawing. Cover the whole drawing except the very whitest parts in diagonal hatching of the very lightest shade on your value chart. Then cover again the whole drawing apart from the lightest and the second lightest areas with another light layer of hatching, this time working at opposite diagonals. Then leave the 3rd, 2nd and whitest areas when you do your vertical shading and so on. By doing this you are working on your drawing as a whole rather than getting too involved with detail too soon. Hatching lines can also curve and follow the contours of an object.
It’s easier to hatch one way than the other, depending on whether you are left or right handed, it can help to turn your paper upside down so you are always going in the easiest direction. Big hatching strokes are really difficult to get good at and it take practise to improve rhythm and consistency of line.
Build up the darker areas by repeating the hatching marks...two diagonals, a vertical and a horizontal on top of the four directions you have done already. Work into it for as long as you want depending on what level of realism you are trying to achieve.
Use your rubber as a tool by hatching and blending as you would a pencil. I cut off sharp angled slices to give me a good edge to work with.
Try holding your pencil normally then holding it between your thumb and finger, like a toothbrush for a softer finish.
Vary your pencil pressure according how dark and light the lines are as well as for the shading, it makes a massive different to the look of a drawing.