Argumentative essay

I have no idea about writing an argumentative essay because it contains a huge knowledge. For example, eating fast food should be avoided. Of course, we have enough information to talk about that topic, and we can control the whole essay. However, if we don't know in general, we will not discuss in the good way as we thought. I have a trouble with any kind of essays because i am cramming myself with the topic and the vocabulary because I have lack of words in English to explain; moreover, I am afraid of teacher who read my essay. I try to improve in writing so much, but sometimes I get a good grade and some are not. I am really upsetting about myself. Tomorrow I will take an exam, and it is not easy. Otherwise, I will do my best.