Charcoal is a forgiving medium which is used for tonal work. Easy to rub out and work back into to alter tonal areas. Charcoal tends to rub off on everything and needs to be fixed to the paper with a spray fixative such as airborne glue. Avoid getting it near your watercolor work. 

This sketch was drawn by Janice Anne one evening with nothing to do.  Though spontaneous whim sickle visions she has drawn simply what came to her.   Dreaming up what you want is often a difficult decision.  In this case she just sat down and started drawing.  Before you begin sketching, get some knowledge of different types of pencils there are.  Click Here for a list of Materials needed.