There are pros and cons to starting woodturning as a pen-turner. In its favour are factors like the limited number of tools that you'll need, the small amount of timber that you'll be turning, but against it are factors such as the precision needed to make pens and the relatively low level of experience that you will gain. Woodturning is a massively wide discipline, so it seems a shame to specialise from the outset.
My best advise would be to find a woodturning club near you and get some hands-on experience with people who can guide you through the various skills and techniques. You might also consider finding a professional nearby and getting some professional training....there really is no better way to get experience.
Les