Image found at mustseecinema.com
“I became quite successful very young, and it was mainly because I was so enthusiastic and I just worked so hard at it.”
Don’t fool yourself into thinking that screenwriting is glamorous. It’s work. Hard work. And possibly one of the hardest things to do is to actually start writing. Screenwriting is technical, it's artistic, and it is definitely satisfying. But, it requires concentration and focus that is unmatched in filmmaking. That is why it's so important to be energetic, to be excited by the inherent challenges screenwriting throws your way.
There are any number of distractions out there, but by simply starting to write something, anything - you soon gain momentum and are able to put the distractions aside. So, shut off your phone. Unplug the network/internet cable from your computer, and start shaping a story.
Choose subject matter that is engaging and important to you, for you will spend countless hours slogging away at this. Make sure it excites you, personally - because after a few weeks, it'll get monotonous. But, don't delay. Don't plan it all out in your head. Don't outline it into oblivion.
Just get started already!