To be able to call yourself a writer there’s only one thing you need to do. Write. Sounds easy right?
To be a pro writer though, you need to write every day. Whether or not you're inspired. Irrespective of whether you "feel like it". Every single day. No exceptions.
Suddenly it doesn't sound quite so easy...
This article covers everything you need to do to turn yourself from a writer (or even a wanna-be-writer) into a pro writer.