Take a look at the overall fall of light. Since this is a sketch, not a detailed drawing, I'll do a bit of rough shading to indicate the shadows. I can see that the light is coming from top left, making the lower right side slightly darker. There are also shadows under the dog's ears.
Look for your highlights. Be sure to keep the highlights in the dog's eyes. 'Reserving' means leaving the white of the paper - sometimes it can be hard to erase pencil, so you avoid shading those areas. You'll also find highlights on the nose, and on the fur if the light is bright.
Note where there is white fur. Shading 'in reverse' towards the light fur will create a good texture.