Okay kiddo, a long time ago, there used to be lots of different groups of people living in what's now called Mexico. One group of people, called the Aztecs, were really powerful and had a big empire. But then, some people from Spain came to Mexico and started fighting with the Aztecs. Eventually, the Spanish won and took over the Aztec empire.
But, after a while, the Mexican people started to get upset with the Spanish rulers. They wanted to be independent and have their own country. So, a man named Agustín de Iturbide decided to lead a rebellion against the Spanish rulers. He got a lot of support from other people who also wanted independence.
Finally, after a lot of fighting, they won and became their own country. Agustín de Iturbide became the first emperor of the new country, called the First Mexican Empire. But, unfortunately, he only ruled for a short time before he was overthrown by another rebellion.
So, that's how the First Mexican Empire was formed and how it ended. It was a time when the Mexican people fought for their independence and became their own country with their own ruler.