The previous post was very strong. Squealer's control of information and how people receive it made him indispensable to Napoleon's ability to consolidate and exercise power. His "spin" was a great take on the Soviet reading of history and the idea that there was no such thing as a free press. Yet, I think that Squealer also holds some broader implications about political power and information. The idea of "turning black into white" is a magical condition and something that can help all political leaders that are either seeking to gain power or to maintain it. Squealer becomes vital in being able to ensure that the pigs remain in power by skillfully and artfully being able to convince the other animals of the pigs' agenda and running verbal misdirections. Orwell might be suggesting that all media must be vigilant of their manipulation at the hands of those in political power. In the end, Squealer is not only someone that represented the Soviets' use of the media, but any political body that seeks to control the minds of its people through the control of information on both past and present. This is where his role might acquire great significance and meaning.
One of the best places to start is actually in the book itself or in any decent commentary about it. Orwell very carefully created his characters in order to mimic certain political figures of the time. Squealer is no different.
In this case, Squealer functioned as the propaganda arm of the Soviet machine, changing events around so that they will fit with the idea of communism instead of simply being power grabs and Napoleon getting whatever he wants.
You could find some parallel events or stories from the Soviet propaganda machine and link them up to the events on the farm that Squealer changes around, this could really be a fun essay.
Good advice from the previous poster. Another place to start is to make sure you have a good working knowledge about what propaganda is and how it's used. This is a symbolic novel, as you know, and Squealer's role is to be the master of propaganda. (He is one who always convinces the other animals that things are good, that they're imagining things about the past, that the commandments didn't change...and more of the same.) So, I would think an understanding of propaganda, at least at a basic level, would help you understand more about Squealer. Consequently, you'll write a better essay. Happy writing!