Let's first point out a common mistake. You don't need an idea or a legitimate reason to write. So get rid of that ridiculous thought.
What do you actually need?
Im assuming you're a fairly typical human being. (I'm not delving very deeply into exceptions in this article. If you feel excluded, try to find advice on how to feel offended.)
The first thing you need are hands and especially fingers. How do you find out if you have them? The most common way for a human being observing the environment is using it's eyes. Try to find approximately ten adjacent items pointing in roughly same direction. Human fingers are quite ugly, but don't let the scary sight stop you yet.
Ok. So you're confident that the requirements so far have been met. What next?
Let's say you have a keyboard. (If you're using a filthy touch screen the following advice may still work for you.) You know those strange devices consisting of rectangular pieces with cryptic writings on them? Move your hands back and forth like you're poking holes in the air, except this time replace the air with your keyboard. Things will now appear your screen. (Again, a rectangular device, but a little larger one this time.) You have to use your damn eyes again to see the result. At this point you may realise writing is mostly a physical exercise. I said this just to discourage you.
You know the story of infinite monkeys and a typewriter? Acquiring that many monkeys is probably overkill for this task, so we'll ditch the poor monkeys.
How do you know if you're done?
This is easy. You only need to know two things.
1. Close your eys a bit so your vision is slightly blurry. (If you're into extreme sports, walk away from the screen far enough so you can't distinguish individual letters.) Now, do you have enough black on white for this to have a striking resemblance of an actual article?
2. Are you bored or tired enough to stop?
If you, unlike me, actually like writing, this article may not be for you. So why are you still reading? Do you like reading too??