It's just a pencil but it plays light and shadows.Basic tips for the beginners to turn the sketch around and give it life are:
Direction of light: Begin with the knowledge of direction of light. If the source is a photograph or real life, figure out where you can see light coming and make sure you have it in the sketch in the same spaces.
Transition to shadows: Once the direction of light is determined, its time to figure out dark and light shadows and spot them to sketch in a similar way.
Blending and smoothening: Light and shadows need to blend in a way that it looks natural. so it goes like light, dar. darker and darkest.
Work in layers: Pencil marks make the painting lose its essence. So if you want to skip the layers and straight away want to paint the last one, any erasing just in case you make a mistake will have a sad texture to the sketch base. So start with very light base and develop it in layers.
Choice of pencil: While working in layers, always go from lighter to darker pencil (e.g. from 4B to 8B). This make you make changes without ruining the sketch