There is a noble and gracious cry arising in the ranks of the author-internet universe: if you write, you're a writer!
As far as the dictionary is concerned, this is true. Writer = one who writes. If you write sometime, regardless if it is thick novels, short blog posts, or a free-verse poem, you are a writer. You have accomplished something.
However, it is also a title that gains weight and status alongside how seriously you pursue it. When an individual claims to be a runner, you probably automatically assume that they, at the very least, have completed a 5k. But if they were to tell you that they simply ran out to their mailbox, they would still be correct in their self-definition, because a runner is one who runs. If someone tells you that they are a cook, and yet they consider themselves so because they made Kraft mac and cheese, they are also correct, if you make such a statement equivalent to your own as a writer.
All this to say that, yes, you are a writer! But in order for such a label to become more serious, you must work more extensively in your craft. To be a writer, one must write! And it does take practice. No one can run a marathon without training, and no one can simply prepare a gourmet meal without having learned how, so know exactly what your goal is with your writing, and set out to reach it.
But at the same time, remember, as long as you are etching words, whether on paper or on a digital document, that you are writing. And therefore, you are a writer.