The title is sometimes self granted and sometimes bestowed by Wiccan elders in a defined path of Wiccan tradition. There are no absolute requirements so it is easy to claim the title with little to substantiate the claim. However many HPs will have the backing of those they lead and will have risen to the position by merit so although the title does not necessarily imply a thorough grounding in Wicca in many cases it is a genuine recognition of ability.
Some Wiccan traditions operate on a system of "degrees" which corresponds to a path of learning culminating in the third degree which is the level of HP. Again there is little/no consistency between different Wiccan paths and third degree will have differing meanings depending on the group and the tradition.
I am not a High Priestess, no. I work as a solitary, I have no "qualifications" of this nature and were I to ever branch out to lead a coven I would not choose to title myself HP because I am not a Wiccan. To be honest I'm not keen on the term, it can be a little divisive and set up a hierarchy that in my opinion is more suited to commercial business than the practise of the craft. If I ever run a coven (and yes, I do occasionally consider it) it would be more King Arthur's round table and less Chief executive wielding all the power.
To be fair though a lot of HPs don't see the role as being about power but as being about supporting, helping and encouraging others. These types are undoubtedly of great value to the covens they represent.