An argumentative essay is one where you state a thesis and support that thesis from the text of the poem or other relevant sources.
For an essay to be genuinely argumentative, it must make a point about which readers can legitimately disagree. Part of the essay, in fact, should include counterarguments someone might make against your thesis and refute those potential counterarguments. Thus, for example, you could not write an argumentative essay claiming that Wordsworth used the sonnet form in the poem. Since no one would disagree with this claim, that would be a demonstrative rather than argumentative essay. Instead, the central thesis should be one about which scholars disagree. A good starting point for discovering scholarly disagreements is going to the MLA database and reading critical works on the poem.
One starting point for an argumentative essay might be the lines:
Great God! I’d rather be
A Pagan suckled in a creed outworn.
Although Wordsworth's attitude towards nature is often described as almost pantheistic, he actually was a devout member of the Church of England, and these lines could be held to reflect that issue. For a thesis, you could choose one side or the other of the argument over whether this poem represents Wordsworth being sympathetic to a sort of neopaganism or whether it should be read through a lens of traditional Anglican theology. You could use a combination of textual and biographical information to support your point.