Why do you want to start experimenting with pastels by choosing the most difficult of subjects: portrait?
Unless you are very experienced in portraiture I suggest you try a simpler subject: landscapes.
As I often say to my students " a crooked tree nobody notices, but a crooked nose...everybody see"
Start with an easy subject, get the feel of the medium, try different supports, different strokes, layering, direct painting, colored grounds...and have fun. The best way to keep painting is to have some degree of success. If you start with a difficult subject, a new medium, a large piece, a complex composition...you will probably get frustrated not because pastels are not the medium for you, not because you are not a good artists....but because you put too many things on your plate.
Start low and aim high....