Cases like this one should help you develop a keen eye for evaluating sources effectively. When you encounter a text that supports or contradicts your working thesis, your first step is to decide whether or not its argument is persuasive. Does it provide firm evidence for its claim of twenty years or does it simply guess? The journal is peer-reviewed, so you know that its author is a reputable scholar in this field. What other sources does the article cite? Are they reputable too? Double check a few of them to see if they make sense. See section 6 for more details.
Once you have chosen and refined a topic, you will need to form a set of research questions about that topic, and next form a working thesis to answer the research questions. But what exactly is a working thesis? It is a proposed answer to a focused research question, and it is the main point of your argument that you develop throughout your paper. A working thesis is "working" because it guides your research at the same time that your research tweaks it. A working thesis is far enough along to serve as a viable research question-and-answer-pair, but it is still pliable and open to being altered or refined further as your research progresses and as you discover other, related research questions and answers.
Choose a topic and formulate a research question and working thesis.
Finally, students can put forward a proposition. For instance: Snyder builds his poem on nouns to give power to the "things" in his scene. Or Snyder chooses verbs that seem to yield to the nouns in order to tell us how to behave in the presence of nature. This proposition, with some tweaking, can become a working thesis.