The rhetorical situation

It's always hard to start a writing assignment if you're only writing it because it is an assignment. But writing assignments are just practice for real world writing (or speaking, or producing media, etc.), and real-world writing, when it occurs organically, is almost always provoked by something that happens. Some situation catches the writer's attention, and the writer feels the need to alert others to that situation. When you know WHY you are writing, and WHO you want to read it, it will be a lot easier to know WHAT to write.