As noted above, a noun is a word that represents a person, a place, or a thing. Nouns can be either abstract or concrete. Abstract nouns try to briefly represent a concept, whereas concrete nouns are more specific. Because this is a long poem and because Coleridge is trying to paint vivid visual images, he uses many concrete nouns that help us experience what the Mariner experienced. What can make this exercise confusing is that the same word in English can function as a different part of speech, depending on its context. For example, the word love can function as an abstract noun but also as a verb. So we need to look for words that are functioning primarily to denote a person, a place, or a thing, not an action.
Some examples follow. When the Mariner describes his ship entering the cold regions of the antarctic, he describes the area using such nouns as "mist," "snow, and "ice," which a paint a picture of the environment. He goes on to talk about "clifts" (cliffs) and to note that there are no "men" or "beasts" around, just "ice." A few stanzas later, the Mariner introduces the "Albatross," another noun, then "fog," then "food" and "helmsman."
To recap, some nouns used in stanzas 13–17 include: mist, snow, ice, clifts, men, beasts, albatross, fog, helmsman, and food.