How do you start an essay about being a good citizen?

The same way you start any essay - plan.

Look at the instructions for the essay, what are the important Words, what Words tell you what you have to include in your essay.

Write down some ideas about being a good citizen, group your ideas together, these groups could be the basis for different paragraphs.

It is hard to tell you exactly because I don't know the details of what you have to write.