I'd say it would be having an idea pop up that can be expanded on. You do this just as much to clarify the idea in your own head as you do to inform a reader.
It is said that Sir Francis Bacon - a contemporary of William Shakespeare (Some even say that HE wrote the Shakespearean plays) propounded that:
'Reading maketh a learned man, Writing an EXACT man, and discourse, a ready man.' We need to place our emthasise equally across all three of these and we really do become the 'rounded man' educational-wise.
I suggest a lot of reading across a wide range of topics will fill you up with plenty of ideas to draw on. Once you have a few, just start writing. Don't worry too much about the writing's quality apart from determinining to keep editing and improving it. It will improve of its own accord.
Good luck, son.