Suppose you have to speak on 'contribution of science', and that too, in three different places. The topic is the same but the audience is very different: First you have to speak on this in front of the school children, then in front of a General Assembly, and at last, in front of scientists. How would you speak? I hope you have understood what I am trying to say. Your language and style would be different in front of the school children. It will be different in the General assembly and, of course, very different with the scientists. Your language will be very formal and will have scientific terminology with the scientists. That, actually, would be a scientific language, very different and not at all comprehensible for the common man. However, in the general assembly, you will speak not only about what benefits the common man, but, also, what they canunderstand. Wherever you feel that they do not understand something, then, you have to make them understand in a different way. You can use examples, some stories or incidents, some amusing things, discuss incidents, etc. Your efforts should be 'to make them understand anyhow'.
This is what I have called, 'informal style'. This is one of the styles which don't have a fixed pattern or a format. But, this doesn't mean that it is deformed. It is said that the topic itself chooses its style. This way your style depends on the topic you are going to write your essay on. This is for sure that you don't have to show 'how knowledgeable I am' in it.
If you learn by heart this meaning of essay, then, for sure, your fear of writing it, if there is any, will be removed. However, it is very important to practice for the same.