Honestly, that is all it takes to be a writer - writing. Now to be a successful writer, writing needs to become a habit. I've been following both Paul Guyot (@Fizzhogg) and Brian Koppelman (@BrianKoppelman) both on twitter and their own blogs (http://paulguyot.net/ and http://briankoppelman.com/ respectively) for a while now and I have learned so much from them.
All of their advice and wisdom, however boils down to the fact that to be a writer you have to write. Every. Single. Day. Write when its easy and the words just seem to flow onto the page of their own volition. Write on the days when you have no idea what to write about as well.
Don't be intimidated by the empty page. Rather hear its siren call, begging you to put pen to paper and fill it with your words. Those words don't have to make sense at first, write what you feel and what you want others to feel. Write about what you see and hear and smell and feel around you. Its amazing how the slightest prod can turn into some of the most wonderful stories or scenes or dialogue that you've ever written.
Don't get me wrong, just as often as not, when you do this you may get something that's god awful. that's OK too! The process of expressing yourself is a learned behavior and the more you practice it the easier it becomes. Nothing written is a waste of your time. Everything helps you to become a better writer.
So, have you written today? I have. :)