What, organize? Can't I just write out some similarities and some differences? Compare and contrast essays are deceptively simple, but in order to have impact, they require solid planning.
At first glance, you know that the two subjects you'll be comparing have something in common, and you may want to write about it. Don't.
Whether your topic is the Magna Carta versus the Constitution, or Mahatma Gandhi versus Nelson Mandela, you need an angle. Do you want to compare New York and Paris? Solar and wind power? Dachshunds and poodles? You simply won't get enough mileage from your core commonality. It will either be too small -- you can't turn the statement "They're both types of dogs" into a full paragraph. Or it will be too big: if you state "They're both documents that limit the power of the heads of state", you've oversimplified your argument and lost the opportunity to discuss interesting and subtle nuances.
But don't give up! You just need a method to organize your notes and find common areas that are not too big and not too small, but, as Goldilocks would say, "Just right".
Take a look at the details of the topic, and start organizing these based on how they overlap for the two topics. Do you have information about Gandhi's education and Mandela's childhood? Now you're getting somewhere! You can start gathering details for a paragraph that will compare these two leaders' early lives or formative years. How are dachshunds and poodles bred? Which countries use the most solar or wind power?
Find ideas for topics in the information you already have. But don't be afraid to use a great topic that just inspires you, separate from the information you have; you can always add on to your notes after doing more research so that you have enough details.
What will you do with those details once you have them? How will you put the whole essay together once you have those "just right" categories? In this video I take you through a sample topic and outline and answer exactly those questions.