The speaker stands at a threshold, unsure which way to go. Entering the city or leaving it, the opening question is the same: where should I begin? It sounds like someone about to write, or to look closely, and not knowing where to start.
The answer comes in the second stanza: begin with "the particular." In other words, start with one small, specific thing, a single detail rather than the whole sprawling place. The last line adds a quiet hope. Paying close attention to something ordinary might, just possibly, lead to "glory," a moment of wonder or revelation. The phrase "in case" keeps this modest. The speaker isn't promising anything, only staying ready.
The mood is calm, curious, and slightly wry. There's humility in it, and a gentle openness to being surprised. The title, "Observer," fits: the speaker's job is to notice and let the noticing do its work.