Have you ever had to write an essay?
How hard is it to know what to write when you are staring at that blank page?
Where do you start?
How do you say what you think?
How do you say it well?
Start by putting something down - anything.
As sculptors cannot work without the material to shape, you cannot be creative without working with something, on something, shaping it.
From that initial source, those first incoherent lines, you can begin to shape your argument. Not out of thin air but from volume, mass, material - fuel.
Don't worry about it being perfect - you'll see to that further on, you're creative. Just create something and THEN start to get creative.
You'd be surprised about what you'll think by the act of simply putting something into the world. All you need to worry about is starting, the rest is downhill from there.
The act of creation breeds creativity.