Everyone has a different idea about what it means to be a writer. For those with high standards, you can only start calling yourself a writer after your first best-selling book is made into a movie. For others, nothing short of being published through Harper Collins or Simon & Schuster will do.
So when do you get to start calling yourself a writer?
In my opinion, there is only one qualification: you just need to write. You can write poems, diary entries, web site copy, emails, short stories, long stories, marketing blurbs. The specific content matters little. It is the act of writing itself that makes you a writer.
Many amateur writers keep their stories secret, the way they'd hide an affair. It's very scary to share your work, but you ARE still a writer--even if your masterpiece is tucked away in a drawer, unfinished.
Writing is not like psychology, requiring certifications to be able to claim the title. You can practice without a license! It's simply something you proclaim to yourself when your work is in the development stages, and to others when you feel courageous enough to be vulnerable. Not everyone will honor your courage and vulnerability, but no one can take the title of writer from you without your permission.