The first thing that you need to consider is that which grade you will be delivering the lesson to. You cannot be delivering an in detail lecture to students in primary school. Similarly you do not want it to be too simple if you are teaching a Senior High School class.
Know the History
In order to prepare the lesson, you should know about the history and functionality of the organisation yourself. Make sure that you go through the various aspects and the contributions that the organisation has made over the course of its existence. This will give you a great idea of what you want to include in the lesson.
Make the introduction solid and interesting at the same time. It will settle the tone for the rest of the lesson. Make sure that it does not include too little or too much information as it can leave the listen gasping for more or too stressed by what is provided to them.
Break down the body of the lesson into various parts. Try to keep them concise and prepare them in an engaging manner. This will keep the students interested and they will continue to learn as well. It is a win-win situation.
Once the lesson has been delivered, give an assignment to the students. A part of it should be from the lesson and a part of it should be to make them explore something new about the organisation that was not in the lesson. Not only will they learn something new, it will also help them in doing research on their own which is very crucial in this day and age.