I believe anyone can share the dharma, whether to your
non-buddhist friends, or to buddhist friends on the internet or not
on the internet, but to be an 'official', 'qualified' Buddhist
dharma teacher, you should be 'qualified' to teach by your Buddhist
teachers from the respective lineages.
Also when we talk about taking someone as our 'official
teacher'/'guru'/'spiritual guide', my master say we should look out
for 'ming2 shi1' meaning enlightened persons, or someone who has
ming2 xin1 jian4 xing4. That means that the person has entered the
noble path, has realised the true nature of reality, and therefore
is able to guide others towards the noble path as well. Otherwise
he can only talk and discuss, but with no first hand experience and
intuitive insights, he would not be able to closely guide others
along the path to enlightenment.
So definitely if you want to be a dharma teacher then you need to
be highly enlightened yourself, and be authenticated by the
teachers of the various traditions in Buddhism, and of course
received the permission to teach by their teachers.
And other than being highly enlightened, having first hand
experience and intuitive insights, of course you must have certain
knowledge of the dharma, and be able to teach the dharma in a way
that the students can understand and apply them.
There are many more 'criterias' like having strong morality...
right motivation (compassion, and the patience and willingness) to
teach.. etc etc.
Also some good reads would be: SPIRITUAL
GUIDES: Pass or Fail?, Code of Ethics for
Spiritual Guides, THE
BUDDHA AND THE QUALITIES OF A DHARMA TEACHER
