You are more than welcome to invite your writing and aspiring writing friends to visit us here and, in line with fair use, to quote any of my observations with attribution elsewhere.
Let's begin our journey together by beginning at the beginning.
Believe it or not, this question has more than one answer. For instance, if you keep a diary or a journal, you are, I suppose, a writer. If you send emails, or tweet, or write memos or proposals you are, again, in some sense a writer.
Now, a professional writer is someone who gets paid for their writing, but the paid part is a whole other topic.
For our purposes, then, a writer is a person who writes, paraphrasing Stephen King, to enrich the lives of those who will read your work and your own life as well.
So, in the normal course of events, a writer may be a person who writes anything such as books; stories; articles; poetry and light verse; newspapers and newsletters; stage plays and screenplays; greeting cards and cookbooks; children and young adult books; epigrams and jokes; histories; biographies and autobiographies; science fiction, fantasy, horror, humour, mysteries, romances, and scores of other materials.
This may seem blindingly obvious to you, but it was not to me. All I knew was that I wanted to write but wasn't sure if it was for therapy or fun or what.
Start by starting. There is no magic way for any of us to begin, or even continue, our writing journey except to write something. One size does not fit all. Tell a story or report an incident, or relate a historic fact, or compose a poem or [fill in the blank]. It doesn't have to be 'good'. The odds are it will not be good. It will, though, be an all-important start. Rejoice.