One approach to writing an allegory is finding symbolic characters and a setting that will represent the historical event you're writing about. In Animal Farm, George Orwell uses animals for the masses who rebel against the tyrant farmer. And this novel is an allegory for what Orwell saw as the failures of the Russian Revolution.
Plato's "Allegory of the Cave" symbolically describes the way Plato thought we interpreted reality. He suggested that we are chained to the floor, unable to turn around. We watch shadows on the wall of the cave. Those shadows are cast by people behind us dancing in front of a fire. We can't see those people or the fire because we can't turn around. We only see the shadows. Outside the cave is the Absolute Truth. Plato suggests that our reality is thrice removed from the Truth. There is the outside Truth, then the people in front of the fire, and finally the shadows. This was Plato's way of illustrating how we need to do philosophical work in order to even think of that abstract, Ideal Truth.
So, going along with Orwell's idea, maybe you could choose a topic in recent history such as the promise of the Arab Spring. Instead of using animals, maybe you could use workers in a factory who rise up against their oppressive bosses.
Or you could take a more pop culture approach. Write an allegory about the dependence on cell phones and technology. Exaggerate to make your point. For example, have half-human, half-machines walking around, bumping into each other because their gaze is fixed to a smart phone screen. Allegories can often be satiric and comical. Maybe the cell phone machine-people, after bumping into each other, fall down. Since they are too busy texting, lying like turtles unable to get off their backs, they require other machines to come pick them up, and put them back on their feet.