It seems such an obvious thing to say it, but to be a writer you actually need to write something.
It's funny how many people on the interwebs say they write, but who are actually still working on their first project, the very same one they've been writing for the last five years and still haven't got past halfway. Then there are those who actually do finish that one script, send it out to a few production companies, think that's it and sit back and wait for the money to roll in.
The first three years after finishing my scriptwriting degree I fell into the latter category. I had a completed script, sent it out, half started another, but then decided I didn't need to finish it because the first script was going to get sold and then I'd have the time and money to sit down and write. How naive and stupid I was back then. It wasn't until I was in my fourth post university year that I realised I didn't have a writing career, I had exhausted all avenues with my one completed script and I wasn't going to get anywhere unless I got up off my arse (or actually sat down on it), put myself in front of my computer and wrote something new.
I realised I should always be working on my next project and building a portfolio of quality screenplays. Words had to be typed for that to happen. The most terrifying words you can hear are, "Loved your script. Can you send me something else?" only to realise you don't have anything else to send. That's why I have the words - WRITE YOU LAZY EFFER!!! - written on the whiteboard above my desk, as a constant reminder I'm only ever as good as my next screenplay.
So if you call yourself a writer online then make sure you are actually writing...all the time.