First of all, collect information about the Vietnam War and the supporters. Decide if you agree or disagree with them and have the "proof" why. The first paragraph will introduce the theme of your essay and could contain the thesis statement.
The next paragraph (or more, if you wish) contains the body of the essay and the last paragraph will be the conclusion.
For information, try some of the following links:
How to write an essay:
When you are finished, if you post it here someone will critique it for you.