Writing and reading are intimately related. As you start to get clearer about writing a thesis statement, you will also get better at identifying topic, thesis, and thesis support in any essay or book that you are reading.
Argumentative essays are best approached in levels. This means that rather than reading from beginning to middle to end (like a novel), you should first quickly inspects the essay (title, opening paragraph, concluding paragraph) to find the topic and (hopefully!) the thesis statement that will be defended. For example, an essay with the title “Why Dogs are better than Cats” is probably a dead give away. Then, you should look to see whether the essay is divided into sections or parts. You should also look to find the most general support or argument for the thesis. For example, the essay might list five reasons why dogs are better than cats (each reason elaborated in a few paragraphs). This would give you an essay with five major parts (plus intro and conclusion). Then, you can look at each part of the argument to see if there are sub-arguments, and so forth within each section. You get the idea!
When you read an essay with this agenda in mind (what is the topic, what is the thesis, what is the argument), not only is the essay more easily (and quickly) digested, you are encountering fine examples of how to write thesis-defense essays. You will pick up good strategies and techniques along the way. Your own writing will improve!