Form a thesis. As you will be expected to weigh the pros and the cons about your thesis all through your essay, be able to defend it with clear and factual arguments. Your thesis can, for example, be, "The most memorable poems were written in the romantic era."
Start with arguments that support your thesis. You are trying to convince your reader to believe you, so you should begin the essay with facts that speak in favor of your thesis. The before-mentioned thesis can be supported by mentioning a few famous poets from that era. Name three or four and explain why they are remembered.
Convince your reader further. You will need at least three arguments that support your thesis for a good argumentative essay. Poems in the era of romantics are known for the richness of syntax, so you can write about how the poets structured their poems in that era and explain why that makes the poems good.
Connect your thesis to the present. The best way to convince the reader your thesis is correct is to present it as still relevant. The themes often used in romantics were love, loss and nature, as well as self-doubt and self-questioning. Explain how those themes are timeless and how many people still relate to them today.
Find an opposing idea. Although it may seem counterproductive, the structure of an argumentative essay needs an idea that recognizes differing viewpoints. When you are writing about poetry, you can pick another poet, poem or era that also carries significance today and describe why others feel they are memorable.
Conclude with the main ideas. Go through your essay and sum up the main points you made, including your main thesis in the conclusion of the essay. Keep the conclusion short and describe each idea with only one sentence.