Learning to draw portraits is fundamental to being able to paint portraits. The best way to get started, is to learn from professional artists.
Copying drawings is a worthy exercise and very different to copying nature. Seeing how artists use line and pencil marks to make their drawings alive and expressive will teach you how to do the same.
Drawing by observational complexity alone does not make great drawings. Drawings need to stand on their own as interesting artworks and this means seeking out poetic and free flowing pencil lines and using a variety of pencil marks to describe the three dimensionality of form.