To write an argumentative essay firstly you must set out guidelines for your topic.
Start with an introduction and a hypothesis.
Keep your hypothesis direct and to the point. For example:
"Cars have contributed to the greenhouse gases"
Not,
"Ever since the first car was manufactured a few decades ago in a small factory, the car ran on man power back then but then changed to petrol, the effects of global warming, a major issue for our planet, have been greatly changed due to cars that a driven by people, that's you and me!."
Then once you have written your hypothesis and introduction you would plan out your argumentative points.
Use a paragraph for each point.
When writing these paragraphs start with a topic sentence.
This sentence outlines what you will be arguing in this paragraph.
Then back up this sentence with sources and factual information.
Then write about the reliability of the source you used, and here you may either come to the conclusion that the source you used was accurate and representative or you may find it wasn't accurate.
Now you would bring in a contradictory source to prove that the latter source is more accurate.
Now you would write a sentence that would hint at the topic of the next paragraph, this is called a linking sentence.
You can use that paragraph structure for each paragraph and argumentative point that you will write.
No we come to the conclusion.
Usually conclusions start with these words:
"Therefore it can be seen"
"Thus it can be said that" - insert your hypothesis here, but don't insert it word for word, change it slightly so it isn't an exact copy.
After this you're finished

.
G'luck.
Those are the very basics of argumentative essay writing, if you want more help pm me i'll get back to you with much more info.
RowMac