I agree with the second answer that writing is a passion which one appears to almost be born with; something that 'has to out.' Yet I also find agreement with the idea of early practice, that minimum of 100 words per day until a habit is formed. If you ensure that you do write those 100 words or so every day for a month, you'll likely keep it up for life.
There may be breaks, of course, but you'll find that you just have to get back into it. For as life presents you with those things that effect your emotions, you'll hardly be able to contain yourself. Novels, short stories, essays, poetry come from that urge. Give into it. You'll not only feel the better for it, it will enable your growth as a human being.