For any kind of pencil sketching, the initial requirement are pencils of variouus kinds. The initial drawing is done with an HB pencil and the details are put using HB pencils of higher intensity.
I would like to share this secret with you, the areas of darkness are done using the highest intensity shade as you can see under Ganesha's right arm and around the ears.
Just keep on using your imagination and doing any amount of work on the sketch you want to draw.
The result is really going to be spectacular. All the best and keep going.