There are several things I try to bear in mind when coming to write a French essay- some which apply to essay writing in general, and others specific to writing in French. Firstly I have found that it is important to not state your conclusive argument in your introduction, but rather to leave it for the end of the essay. As for the main body of the essay, from my experience it is better to write one succinct point per paragraph, as this keeps your argument focused.
It is also a good idea to take note of useful phrases that you may hear or read in class, which can later be used in your essay. I have also found that taking the time to learn different ways of expressing your opinion in French such as through the use of impersonal verbs- 'Il est fort probable que' or simply using phrases other than 'Je pense que' can make your essay sound more convincing to the reader.